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Johnson Controls Software Engineer Interview Guide

Johnson Controls Software Engineer interview guide: reported rounds, questions and how candidates describe the process.

The 60-second brief

As a Software Engineer at Johnson Controls, you will write software that bridges the physical and digital worlds. Johnson Controls is a global leader in smart buildings, energy efficiency, industrial refrigeration, and integrated fire and security infrastructure. The software engineering team develops critical platforms that control building automation systems (BAS), manage complex HVAC networks, run modern chillers, and orchestrate smart building IoT ecosystem applications like Metasys. In this role, your code directly influences how large-scale industrial systems, commercial high-rises, and critical facilities operate. You will be responsible for designing and deploying resilient backend microservices, robust firmware, intuitive user interfaces, and embedded control logic.

This guide is scoped to a Software Engineer candidate at Johnson Controls.

Focus areas

  • Electrical Machines (Induction machine, motors)
  • Transformers (working, types)
  • PLC (Programmable Logic Controller) Fundamentals

The Johnson Controls interview process

Johnson Controls candidates report 4 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

Online Screening Assessment

Initial assessment consisting of aptitude and technical quizzes for university hires or coding challenges for experienced roles.

What to demonstrate

  • Initial assessment consisting of aptitude and technical quizzes for university hires or coding challenges for experienced roles
  • Depth in Electrical Machines (Induction machine, motors)

How to prepare

  • Answer aloud and timed: Explain the core principles of Object-Oriented Programming (OOP) and give an example of inheritance versus composition.
  • Answer aloud and timed: What is the difference between var, let, and const in JavaScript, and how does scope work in dynamic languages?

Technical Round 1

Focuses on fundamentals, resume review, and discussion of past projects.

What to demonstrate

  • Focuses on fundamentals, resume review, and discussion of past projects
  • Depth in Electrical Machines (Induction machine, motors)

How to prepare

  • Answer aloud and timed: How do you implement dependency injection in backend frameworks, and what benefits does it bring to testability?
  • Answer aloud and timed: Write a function to reverse a singly linked list and discuss its time and space complexity.

Technical Round 2

Deeper evaluation of system architecture skills, domain-specific knowledge, and coding proficiency.

What to demonstrate

  • Deeper evaluation of system architecture skills, domain-specific knowledge, and coding proficiency
  • Depth in Electrical Machines (Induction machine, motors)

How to prepare

  • Answer aloud and timed: How do you check if two strings are valid anagrams?
  • Answer aloud and timed: What are common design patterns you have implemented in previous projects (e.g., Factory, Singleton, Observer)?

Managerial and HR Round

Discussion of behavioral attributes, salary expectations, and cultural alignment.

What to demonstrate

  • Discussion of behavioral attributes, salary expectations, and cultural alignment
  • Depth in Electrical Machines (Induction machine, motors)

How to prepare

  • Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
  • Re-read the description of the managerial and hr round above and write down what you would ask to confirm before it.

Where candidates lose points

Prepare clear, structured examples using the STAR method (Situation, Task, Action, Result) for behavioral discussions

Focus heavily on technical decisions, team collaboration, and measurable outcomes.

Master your resume projects

Interviewers consistently base questions on your listed experience. Be ready to explain your exact contributions, architectural trade-offs, and technical solutions for every project listed on your resume.

Highlight low-level or hardware-adjacent knowledge

If you have experience working with sensors, serial communication, embedded systems, microcontrollers, or multithreaded logic, explicitly highlight these experiences during technical discussions.

Emphasize systematically solving problems

When working through coding or system design questions, articulate your reasoning out loud. Interviewers value structured thinking, attention to edge cases, and root-cause analysis skills over speed.

Questions & practice

Explain the core principles of Object-Oriented Programming (OOP) and give an example of inheritance versus com

Medium · Core Software Engineering & Data Structu

Explain the core principles of Object-Oriented Programming (OOP) and give an example of inheritance versus composition.

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

What is the difference between var, let, and const in JavaScript, and how does scope work in dynamic lan

Medium · Core Software Engineering & Data Structu

What is the difference between var, let, and const in JavaScript, and how does scope work in dynamic languages?

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

How do you implement dependency injection in backend frameworks, and what benefits does it bring to testabilit

Medium · Core Software Engineering & Data Structu

How do you implement dependency injection in backend frameworks, and what benefits does it bring to testability?

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Write a function to reverse a singly linked list and discuss its time and space complexity.

Medium · Core Software Engineering & Data Structu

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

How do you check if two strings are valid anagrams?

Medium · Core Software Engineering & Data Structu

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

What are common design patterns you have implemented in previous projects (e.g., Factory, Singleton, Observer)

Medium · Core Software Engineering & Data Structu

What are common design patterns you have implemented in previous projects (e.g., Factory, Singleton, Observer)?

Approach

  1. Restate the input: its shape, its size, and what is guaranteed about it.
  2. Name the brute-force solution and its complexity before improving on it.
  3. Choose the data structure from the access pattern, not from familiarity.
  4. State the target complexity and say which constraint rules the naive version out.

Follow-up

  • How does this change if the input no longer fits in memory?
  • What is the worst case, and how likely is it on real data?

Explain how garbage collection and memory management operate in runtime environments like Java or.NET.

Medium · Core Software Engineering & Data Structu

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Explain the difference between a mutex and a semaphore in an operating system or RTOS environment.

Medium · Embedded Systems, Firmware & Hardware In

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

How do interrupt service routines (ISRs) function, and what design practices prevent race conditions in hardwa

Medium · Embedded Systems, Firmware & Hardware In

How do interrupt service routines (ISRs) function, and what design practices prevent race conditions in hardware-driven software?

Approach

  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Replace offset paging on the resource feed with keyset

Easy · keyset pagination · composite index · sort order

resource holds resource_id, tenant_id, owner_user_id, title, body_ref, version, status ('draft','active','archived','deleted'), created_at, updated_at, deleted_at, with an index on (tenant_id, status, updated_at DESC, resource_id DESC). The listing endpoint returns active resources for one tenant, newest update first, 50 per page, today with LIMIT 50 OFFSET n. Tenants reach page 400 and rows are created while they read. Write the keyset query, define what the cursor carries and how it is encoded, and say which part of the index each predicate uses. Assume PostgreSQL 16.

Approach

  1. Name the two failures separately. OFFSET 20000 makes the server produce and discard 20,000 rows, so page cost grows with depth rather than with page size. Independently, any write that changes how many rows sort above the offset moves the window between two fetches, and the direction decides which anomaly you get: an insert lands at the head of updated_at DESC and pushes already-returned rows down past the boundary, so they are returned a second time; a delete above the offset, or a row whose updated_at is bumped above the cursor, pulls rows up and one is never returned at all. Nothing in the response reveals either.
  2. Write the seek: WHERE tenant_id = $1 AND status = 'active' AND (updated_at, resource_id) < ($2, $3) ORDER BY updated_at DESC, resource_id DESC LIMIT 50. The row-value comparison is one index range rather than a disjunction, and both columns are NOT NULL, which is what makes that comparison well defined.
  3. Map each predicate onto the index: tenant_id and status are equality on the leading columns, (updated_at, resource_id) is the range, and the ORDER BY matches the index order so no Sort node appears and the scan stops after 50 rows. The DESC in the definition only matters for mixed directions — a plain ascending btree on the same columns is read backwards for this query.
  4. Put both sort columns in the cursor and nothing the client can tamper with into another tenant: base64 of (updated_at, resource_id), validated server-side, with tenant_id taken from the principal.

Follow-up

  • The client asks for 'jump to page 400'. What do you offer instead, and what does the honest version cost?
  • Sort order becomes user-selectable across four columns. How many indexes is that, and which would you refuse to add?

Find version gaps and relay lag with window functions

Medium · window functions · event tables · frame semantics

outbox_event holds event_id, aggregate_type, aggregate_id, aggregate_version, event_type, payload, status ('pending','published','dead'), attempts, created_at, published_at. A projection is missing rows and you must decide whether the relay skipped events or the consumer dropped them. Write three queries over the last seven days: one listing every aggregate_id whose published aggregate_version sequence has a hole, one giving per-day counts with a running total, and one returning the newest published event per aggregate. For each, say where the window function is evaluated relative to WHERE and LIMIT. PostgreSQL 16.

Approach

  1. Gaps: compute lead(aggregate_version) OVER (PARTITION BY aggregate_id ORDER BY aggregate_version) in a subquery, then filter next_version <> aggregate_version + 1 in the outer query. Window functions are evaluated after WHERE, GROUP BY and HAVING and before the outer ORDER BY and LIMIT, so the predicate cannot sit in the same WHERE clause and PostgreSQL 16 has no QUALIFY.
  2. Say what the seven-day filter does to the answer: it truncates every partition, so the first row per aggregate has no predecessor inside the window and a hole spanning the boundary is invisible. Widen the window, or join to resource.version as the authority for the true maximum.
  3. Running total: SELECT date_trunc('day', created_at) AS d, count() AS n, sum(count()) OVER (ORDER BY date_trunc('day', created_at) ROWS UNBOUNDED PRECEDING). An aggregate inside a window call is legal because grouping runs before windowing. The grouping key is unique per row here so ROWS and RANGE agree, but write the frame anyway — over ungrouped rows with tied timestamps the default RANGE frame pulls in every peer row and the total jumps.
  4. Newest per aggregate: DISTINCT ON (aggregate_id) ... ORDER BY aggregate_id, aggregate_version DESC is the cheap PostgreSQL-only form when an index matches that order; row_number() OVER (PARTITION BY aggregate_id ORDER BY aggregate_version DESC) = 1 is the portable form and needs a subquery for the same evaluation-order reason as the gap query.

Follow-up

  • Relay failover redelivers events. Does a duplicate break the gap query, and how would you detect one from this table alone?
  • Turn the gap check into a continuous monitor rather than a query someone runs after an incident. What does it watch?

Describe basic circuit components: how do pull-up/pull-down resistors work in embedded logic circuits?

Medium · Embedded Systems, Firmware & Hardware In

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

What is ladder logic, and how are PLCs integrated with higher-level software architectures?

Medium · Embedded Systems, Firmware & Hardware In

Approach

  1. Fix the scope first: who calls this, how often, and what they do when it fails.
  2. Name the read and write paths separately; they rarely have the same bottleneck.
  3. Choose a partition key and say what query it makes expensive.
  4. State the consistency you need, and where you are willing to be stale.

Follow-up

  • What breaks first when traffic grows ten times?
  • How does this behave when that dependency is down for an hour?

Explain basic signal conditioning and how analog signals from sensors are digitized for software processing.

Medium · Embedded Systems, Firmware & Hardware In

Approach

  1. Say who the caller is and what they do when the call fails halfway.
  2. Define the identity of a request so a retry cannot double-apply it.
  3. Separate accepted, pending, failed and confirmed; they are different facts.
  4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

  • What happens if the caller retries after a timeout?
  • How does a client discover it is on an old version of this contract?

What is a PID controller, and how do the Proportional, Integral, and Derivative components work together to st

Medium · Control Systems, Industrial Protocols &

What is a PID controller, and how do the Proportional, Integral, and Derivative components work together to stabilize a system?

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Explain the operational difference between an open-loop and a closed-loop control system.

Medium · Control Systems, Industrial Protocols &

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

How do Air Handling Units (AHUs), Chiller Plants, and Variable Air Volume (VAV) controllers interact through s

Medium · Control Systems, Industrial Protocols &

How do Air Handling Units (AHUs), Chiller Plants, and Variable Air Volume (VAV) controllers interact through software interfaces?

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

What network protocols and topologies are commonly used to interconnect field sensors, controllers, and superv

Medium · Control Systems, Industrial Protocols &

What network protocols and topologies are commonly used to interconnect field sensors, controllers, and supervisory software (e.g., Modbus, BACnet, IP networks)?

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

Describe the ladder logic diagram required to control a motor startup and shutdown sequence safely.

Medium · Control Systems, Industrial Protocols &

Approach

  1. Clarify what is being asked and what a complete answer contains.
  2. State your assumptions explicitly before working the problem.
  3. Say what you would check first and why it is the highest-information step.
  4. Work from the requirement backwards to the design.

Follow-up

  • What assumption would you test first?
  • How would you know your answer was wrong?

What causes a segmentation fault in C/C++, and how do you diagnose memory leaks in low-level applications?

Medium · Embedded Systems, Firmware & Hardware In

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

How do you approach root cause analysis when a critical issue or system defect occurs in production?

Medium · Behavioral & Team Collaboration

Approach

  1. Establish what changed and when, before forming any theory.
  2. Pick a bisection that eliminates candidates whichever way it turns out.
  3. Check the instrumentation before believing the symptom.
  4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

  • What would you look at first, and what would it rule out?
  • How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Johnson Controls candidates report.

Day 1 — Map the Johnson Controls loop

  • Write out the reported sequence: Online Screening Assessment, Technical Round 1, Technical Round 2, Managerial and HR Round.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 4 reported rounds, with the weakest marked.

Day 2 — Work Electrical Machines (Induction machine, motors)

  • Spend the session on Electrical Machines (Induction machine, motors), which Johnson Controls candidates report being tested on.
  • Write one worked example in Electrical Machines (Induction machine, motors) and time yourself on it.

Deliverable: One timed worked example in Electrical Machines (Induction machine, motors).

Day 3 — Work Transformers (working, types)

  • Spend the session on Transformers (working, types), which Johnson Controls candidates report being tested on.
  • Write one worked example in Transformers (working, types) and time yourself on it.

Deliverable: One timed worked example in Transformers (working, types).

Day 4 — Work PLC (Programmable Logic Controller) Fundamentals

  • Spend the session on PLC (Programmable Logic Controller) Fundamentals, which Johnson Controls candidates report being tested on.
  • Write one worked example in PLC (Programmable Logic Controller) Fundamentals and time yourself on it.

Deliverable: One timed worked example in PLC (Programmable Logic Controller) Fundamentals.

Day 5 — Answer out loud: Core Software Engineering & Data Structures

  • Answer aloud, timed: Explain the core principles of Object-Oriented Programming (OOP) and give an example of inheritance versus composition.
  • Answer aloud, timed: What is the difference between var, let, and const in JavaScript, and how does scope work in dynamic languages?

Deliverable: Spoken answers to 2 reported Core Software Engineering & Data Structures question(s), under time.

Day 6 — Answer out loud: Embedded Systems, Firmware & Hardware Integration

  • Answer aloud, timed: Explain the difference between a mutex and a semaphore in an operating system or RTOS environment.
  • Answer aloud, timed: What causes a segmentation fault in C/C++, and how do you diagnose memory leaks in low-level applications?

Deliverable: Spoken answers to 2 reported Embedded Systems, Firmware & Hardware Integration question(s), under time.

Day 7 — Answer out loud: Control Systems, Industrial Protocols & Automation Logic

  • Answer aloud, timed: What is a PID controller, and how do the Proportional, Integral, and Derivative components work together to stabilize a system?
  • Answer aloud, timed: Explain the operational difference between an open-loop and a closed-loop control system.

Deliverable: Spoken answers to 2 reported Control Systems, Industrial Protocols & Automation Logic question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Describe a time when you faced a difficult technical challenge during a deployment. How did you resolve it?

Medium · Behavioral & Team Collaboration

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Tell me about a time you had to work with a challenging team member. What steps did you take to maintain proje

Medium · Behavioral & Team Collaboration

Tell me about a time you had to work with a challenging team member. What steps did you take to maintain project momentum?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a project on your resume that best demonstrates your technical expertise and personal impact.

Medium · Behavioral & Team Collaboration

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Why are you interested in working at Johnson Controls, and where do you see your career path evolving in five

Medium · Behavioral & Team Collaboration

Why are you interested in working at Johnson Controls, and where do you see your career path evolving in five years?

Approach

  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.

Follow-up

  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?