Maintenance Technician Interview Questions (PLC & Controls Focus), With Model Answers

Short answer: Maintenance technician interviews for plants, warehouses and distribution centres usually cover five areas:

• electrical basics: Ohm's law, three-phase power, motor control
• PLC reading: tracing why an output is off
• a troubleshooting walkthrough
• mechanical knowledge: bearings, belts, pneumatics
• safety, with lockout/tagout (LOTO) almost always asked

Many employers add a practical or written test and behavioural questions. Below are 35 questions in those groups, with model answers you should adapt to your own experience.

Written for industrial, manufacturing and warehouse maintenance roles. Answers describe common practice; your site's procedures, the equipment manuals and the electrical code in your jurisdiction always take priority.

Ownership disclosure: PLC Programming and PLC Simulation Software are run by the same team. We link to our own practice labs where hands-on rehearsal helps.

How maintenance technician interviews are usually run

• Screening call: your experience, shifts, certifications and the equipment you have worked on.
• Technical interview: a lead tech or maintenance supervisor asks the kind of questions below, often with a drawing or ladder printout in front of you.
• Practical or written test: some employers use a hands-on bench test, a timed multiple-choice test, or a mechanical or electrical aptitude test.
• Behavioural round: "Tell me about a time…" questions. Answer in STAR format (Situation, Task, Action, Result).

Interviewers listen for a method, not just the right answer. Say what you would check first, how you would prove it, and how you would stay safe while doing it.

Electrical questions

1. Explain Ohm's law and give a practical use. Voltage equals current times resistance (V = I × R). In practice I use it to sanity-check readings. For example, a 24 V DC coil drawing far more current than its rating suggests a shorted coil. A high resistance reading across a closed contact means the contact is pitted or loose.

2. What is the difference between AC and DC, and where do you find each in a panel? AC alternates direction and is used for motors, heaters and the incoming supply. DC flows in one direction and is typical for control circuits, such as a 24 V DC PLC I/O power supply, sensors and solenoid valves. A typical industrial panel takes three-phase AC in, has a control transformer for 120 V AC control, and a 24 V DC power supply for the PLC and field devices.

3. How do you check whether a three-phase motor is good? First lock out the motor and verify it is de-energised. Then:

• measure winding resistance phase to phase (T1–T2, T2–T3, T1–T3); the readings should be balanced
• insulation-test (megger) each phase to ground at the correct test voltage
• check the mechanical side: does the shaft turn freely, are the bearings noisy?

If the windings and insulation are good, I move on to the starter, the overload and the supply.

4. A motor runs for a few minutes and then trips the overload. What do you check?

• Current draw on each phase against the nameplate full-load amps
• Voltage balance across the phases
• A mechanical overload: a jam, a dragging bearing or an over-tight belt
• The overload setting against the nameplate
• Ambient temperature and ventilation

A single high phase points to supply or winding problems. All three phases high points to mechanical load. I never just turn the overload up.

5. What is the difference between a contactor and a motor starter? A contactor is an electrically operated switch for power circuits. A motor starter is a contactor plus overload protection. See our motor starter vs contactor guide.

6. What does a control transformer do? It steps line voltage (for example 480 V) down to a control voltage (often 120 V AC), so pushbuttons, relays and coils run at a lower, safer voltage, isolated from the power circuit. It is fused on the primary and/or secondary according to the design.

7. How would you test a fuse? Live test: measure voltage across the fuse, using the right PPE and procedure. Near zero means the fuse is good. Full voltage across it means it is open. De-energised test: remove the fuse and check continuity. I also find out why it blew before replacing it with the same type and rating.

8. What are sourcing and sinking I/O? They describe the direction of current. A sinking input provides the path to 0 V, so it needs a sourcing (PNP) sensor that supplies +24 V. A sourcing input needs a sinking (NPN) sensor. Mixing them up is a classic reason a new sensor "works" (its LED lights) but the PLC input never turns on.

PLC questions

9. What is a PLC, and what is the scan cycle? A PLC is an industrial computer that reads inputs, runs its program and updates its outputs, over and over. One pass is a scan: read inputs, execute the logic top to bottom, write outputs, then housekeeping and communications. It matters when troubleshooting, because a signal shorter than one scan can be missed.

10. What do XIC, XIO and OTE mean? They are Allen-Bradley instructions:

• XIC (examine if closed) is true when the bit is 1
• XIO (examine if open) is true when the bit is 0
• OTE (output energise) turns the bit on while the rung is true

Siemens and IEC use normally open and normally closed contacts and coils for the same ideas.

11. Why might a stop button be wired normally closed and programmed with XIC? It is fail-safe. If the wire breaks, the input goes off and the machine stops, instead of the stop button silently failing. In the program, the normally closed field contact is examined with XIC, so the rung is true while the button is not pressed.

12. What is a seal-in (latch) circuit? The start button turns the motor output on, and a parallel contact from the output keeps the rung true after the button is released. The stop condition in series breaks it. Our motor start/stop ladder logic tutorial walks through it.

13. Explain TON and TOF timers with a maintenance example.

• A TON (on-delay) starts timing when its rung goes true and sets its done bit after the preset, for example a jam timer that faults a conveyor if a photo-eye stays blocked for 5 seconds.
• A TOF (off-delay) keeps its output on for the preset after the rung goes false, for example running a cooling fan on after a motor stops.

14. The HMI says a conveyor is running, but it isn't. How do you use the PLC to find out why?

1. Go online and find the rung that drives the motor output.
2. Check whether the output bit is on. If it is on but the output module LED is off, or the LED is on but the contactor isn't pulled in, the problem is downstream: module, fuse, wiring, contactor coil or overload.
3. If the output bit is off, find which condition on the rung is false and trace that input back to its field device.

15. What does it mean when an input LED is on but the bit in the program is off?

• the wrong address or tag is being looked at
• a forced value is in place
• the module or channel is faulted
• (more rarely) a configuration or communication problem with remote I/O

Check forces, module status and the I/O mapping.

16. What is a force, and what are the risks? A force overrides an input or output value regardless of the field or logic. It can make machinery move unexpectedly, and it is easy to leave in place. Follow the site's rules: authorisation, communication with operators, documentation, and removing forces before handover.

17. What would you do before downloading or editing a PLC program?

• Get authorisation and follow the site's change process.
• Take a backup of the current program and compare it with the latest stored copy.
• Understand the change and its effect on the machine.
• Put the machine in a safe state.
• Document the change afterwards.

Many maintenance roles let techs monitor online but not edit. Say so if that has been your experience.

18. What PLC platforms have you used? Be honest and specific: the platform, the software and what you did (monitored, forced, edited, added I/O). If you have only used a simulator, say so and describe what you practised: tracing rungs, timers, counters and fault-finding. To rehearse this, use a browser PLC simulator or the PLC interview prep tracks.

Troubleshooting questions

19. Walk me through how you troubleshoot a machine that won't start.

1. Talk to the operator: what happened, any alarms, what changed.
2. Check the obvious things: e-stops, guards, air pressure, alarms, mode selection.
3. Look at the HMI and PLC to see which permissive is missing.
4. Trace that signal to the field device and test it.
5. Fix the cause.
6. Test with the operator.
7. Document the fix and look for the root cause so it doesn't repeat.

All work on the equipment is done under the site's lockout and electrical safety rules.

20. What is the half-split method? Test at the midpoint of a circuit or signal path to decide which half the fault is in, then repeat. It finds faults in a long series chain, such as a string of e-stops or safety gates, much faster than checking from one end.

21. A photo-eye-controlled conveyor keeps faulting intermittently. How do you approach it?

• Check the fault history for a pattern: time, product or shift.
• Inspect the sensor's alignment, lens, reflector, mounting and cable.
• Watch the input live while you flex the cable.
• Check the sensor's range and the product type. Shiny and clear packages are classic problems.
• Check whether the jam timer preset matches the current line speed.

Intermittent faults usually come down to wiring, alignment or a timing assumption.

22. A 24 V DC sensor isn't switching. What do you check?

1. Is 24 V present at the sensor?
2. Is the sensor's own LED changing state?
3. Is the output wiring correct for PNP versus NPN?
4. Is the signal reaching the PLC input terminal?
5. Does the input LED and bit change?

Each step narrows the fault to the supply, the sensor, the wiring, or the input card.

23. How do you decide between repairing and replacing a component?

• How critical is the equipment, and what does the downtime cost?
• Are spares available?
• How long will a proper repair take?
• What is the failure history?

On a critical line I replace with a spare to restore production, then repair or analyse the failed part. I also record it so we can see repeat failures.

24. Tell me about root-cause analysis. Have you used 5 Whys? Fixing a symptom (replacing a blown fuse) isn't the same as fixing the cause (a chafed cable shorting to ground). A 5 Whys or fishbone analysis, backed by work-order history, stops repeat failures. See our 5 Whys analysis guide.

25. What information do you record after a breakdown?

• the asset
• the symptom
• the cause
• the action taken
• parts used
• downtime
• any follow-up, such as a PM change or a spare to stock

All of this goes into the CMMS work order. Good records make the next technician faster and support reliability work.

Mechanical questions

26. What causes bearing failure, and how do you spot it early? Common causes:

• too much, too little or the wrong grease
• contamination
• misalignment
• overloading
• incorrect installation

Spot it early by listening, checking temperature, monitoring vibration, and watching the trend on your PM checks.

27. How do you tension and align a belt or chain drive? Lock out first. Check sheave or sprocket alignment with a straightedge or laser. Set the tension to the manufacturer's specification, using a tension gauge for belts or measured sag for chains. Too tight wears the bearings; too loose slips and wears the belt. Recheck after run-in.

28. A pneumatic cylinder moves slowly or not at all. What do you check?

• supply pressure and the filter-regulator
• leaks
• the valve: does the solenoid get its signal, and does the valve shift?
• flow controls
• sticking or worn seals
• mechanical binding on the load

See pneumatics basics.

29. What is preventive maintenance versus predictive maintenance? Preventive maintenance is time- or usage-based, such as greasing every 500 hours. Predictive maintenance is condition-based: vibration, thermography or oil analysis tell you when to act. See preventive vs predictive maintenance.

30. How do you read a gear motor or pump nameplate, and why does it matter? The nameplate gives voltage, full-load amps, speed, frame, service factor and ratio or flow. You need it to choose replacements, set overloads, check current draw and confirm the right part is fitted.

Safety and LOTO questions

31. Walk me through a lockout/tagout procedure. Follow the site's written, equipment-specific procedure. The usual sequence is:

1. Notify affected people.
2. Shut the machine down normally.
3. Isolate every energy source: electrical, pneumatic, hydraulic, gravity, stored mechanical.
4. Apply your personal lock and tag.
5. Release or block stored energy.
6. Verify zero energy. Try to start the machine and test for absence of voltage with a proven tester.

When the work is done, remove locks and tags only according to the procedure. See our lockout/tagout procedure guide.

32. What is arc flash, and what does NFPA 70E have to do with it? An arc flash is an explosive release of energy from an electrical fault through air. In the US, NFPA 70E sets out electrical safe work practices, including:

• risk assessment
• approach boundaries
• arc-rated PPE
• establishing an electrically safe work condition

Read what is NFPA 70E and arc flash boundaries. Always follow your employer's electrical safety programme.

33. When is it acceptable to work on live equipment? Only when de-energising is not feasible or would create a greater hazard, under the site's energised-work rules: a permit where required, qualified people, the correct PPE and boundaries. Diagnostic voltage measurements are a common case. The default is to work de-energised.

Behavioural questions

34. Tell me about a difficult breakdown you fixed. Use STAR:

• Situation: a line down on a peak shift.
• Task: restore it safely.
• Action: the steps you took, the fault you found and how you proved it.
• Result: the downtime saved, plus what you changed so it didn't happen again.

Specific numbers and a clear method beat a dramatic story.

35. Tell me about a time you disagreed with a supervisor or operator about a fix or a safety issue. Show that you raised the issue with facts, followed the safety rules even under production pressure, and reached a resolution respectfully. Never describe bypassing a guard or lockout to save time.

How to rehearse the practical part

Reading answers isn't enough. Interviewers often hand you a ladder printout or a schematic and ask you to find the fault. Rehearse it:

• Trace rungs and faults: the PLC troubleshooting simulator has a no-account motor-won't-start diagnostic and guided fault labs.
• Motor control and wiring: try a motor-control lab and electrical troubleshooting.
• Timed test practice: the PLC practice test and mechanical and electrical test practice.

Hiring managers: if you want to give candidates a practical, auto-graded PLC and controls exercise rather than only asking questions, see the maintenance technician assessment page.

A simulator is a rehearsal tool. It doesn't replace hands-on experience with a meter on a real, safely isolated panel.

Frequently asked questions

What questions are asked in a maintenance technician interview?

Expect electrical basics (Ohm's law, three-phase motors, motor starters), PLC reading (contacts, coils, timers, why an output is off), a troubleshooting walkthrough, mechanical topics (bearings, belts, pneumatics) and safety, especially lockout/tagout. Most interviews also include behavioural "tell me about a time" questions.

Is there a test in a maintenance technician interview?

Often, yes. Employers commonly use a written or online multiple-choice test, a mechanical or electrical aptitude test, or a hands-on bench test. Ask the recruiter what format to expect.

How do I answer "how do you troubleshoot" in an interview?

Describe a repeatable method: talk to the operator, check the obvious things and alarms, use the HMI and PLC to find the missing condition, trace it to the field device, test and fix, verify with the operator, and document it. Mention working safely under lockout and the site's electrical safety rules.

How should I prepare for PLC questions as a maintenance technician?

Practise reading ladder logic until you can explain why an output is on or off, and practise tracing a fault from symptom to field device. Learn what XIC, XIO, OTE, TON and TOF do, and how forces and online edits are controlled on site.

Related reading

• Amazon RME technician PLC skills
• PLC programmer interview questions
• Automation engineer interview questions
• PLC certification interview questions
• Maintenance technician training program
• PLC troubleshooting complete guide