How to Screen Diesel Mechanics: 9 Questions and What to Listen For Screening Interview Template

What diesel work have you actually done, and on what equipment: Class 8 tractors, medium-duty straight trucks, vocational units like refuse or dump or mixer, trailers, reefer units? Which engine platforms do you have real hours on (Cummins, Detroit, Paccar MX, Cat, International, Volvo or Mack)? Then walk me through a normal day in your last shop: how many repair orders did you close, and what was the mix of PM, diagnostic, and road calls?

What this assesses: Everybody lists every engine brand, so score the day rather than the list. A tech who ran six PMs and a trailer light repair is describing a different hire than one who closed two repair orders because both were diagnostic, and both are useful depending on which bay is empty. Strong answers name the equipment classes with rough hours behind each, pick the platforms they are genuinely fast on, and put numbers on a normal day. The specific thing to pin down is emissions-era exposure, because a tech whose real depth stops at pre-2007 equipment can be excellent on a mechanical engine and lost on a truck that derates. Ask what model years they worked on most, not whether they have done emissions work, since everybody says yes to the second version. Reefer is close to a separate trade. Transport refrigeration techs are usually hired as reefer techs and often at a premium, so a candidate whose background is Thermo King and Carrier units is either exactly who you need or not who you need at all. Vocational work carries hydraulics, PTOs, and bodies that a highway tractor tech has never touched. Be cautious with a candidate who says they can work on anything, who cannot name a single engine platform they prefer, or whose scope turns out to be tires, lights, and oil changes when the role needs driveline and engine diagnosis. Then calibrate against your fleet instead of against the resume: a dealer-trained specialist dropped into a mixed independent shop will be strong on one platform and slow everywhere else for a quarter, and that is fine if you knew it when you hired them.

Which certifications do you hold right now, and which are current: ASE truck certs and which ones, EPA 609 for A/C work, a state or FMCSA annual inspection qualification, a brake inspector qualification? Do you hold a CDL, and what class and endorsements? Can you produce documentation for each of these?

What this assesses: This question is a records question, not a skill question, so treat the answer as something to verify rather than something to weigh. Know the landscape before you read responses. ASE truck certs are the T series, and the numbering tells you what somebody chose to test on: T2 diesel engines, T4 brakes, T5 steering and suspension, T6 electrical and electronic systems, T8 preventive maintenance inspection, with T3 drive train and T7 heating and A/C alongside them. The medium and heavy truck master credential is a set of those tests rather than one exam. L2, the advanced electronic diesel engine diagnosis credential, has a prerequisite cert underneath it and is the closest thing on this list to evidence that somebody diagnoses for a living. All of them run on a five-year recertification clock, which is why current matters more than held. EPA 609 is the motor vehicle A/C credential, not Section 608, and it gates refrigerant purchase and handling. The inspection side is the part with money attached. Federal rules require the carrier to make sure whoever performs an annual inspection meets the qualification and to keep evidence of it on file, with a separate qualification standard for anyone doing brake work, so a tech who cannot sign inspections limits how independently your shop runs no matter how good they are in the bay. Strong answers name specific certs, say which have lapsed without being pushed, and know whether their inspection qualification was ever documented anywhere. Be cautious with a candidate who says ASE certified with no test numbers, who is vague about expiration, or who assumes an inspection qualification transfers with them. Last, ask about the CDL for a real reason: if the job includes road-testing trucks on public roads, that tech is operating a commercial motor vehicle, and most carriers put them in the DOT random pool and run a [Clearinghouse](https://clearinghouse.fmcsa.dot.gov) query the same as a driver. Confirm that with whoever owns your compliance file rather than guessing at it.

A truck comes in with a driver complaint of low power on grades and a check-engine lamp. There are three stored codes, two of them for sensors, and the driver says it has been getting worse for about a month. Walk me through what you do, in order. At what point do you put a part on the truck?

What this assesses: The order is the whole grade, and this is where a written answer beats a phone call, because you can lay six candidates' sequences side by side on one page. Strong answers verify the complaint first, either by driving it loaded or by duplicating the condition on the dyno or under load, because low power on grades and low power everywhere are different problems. Then they read every module rather than just the engine, and they look at whether codes are active or inactive and how many counts each has, since a code with two counts from last February is not why the truck is slow today. Next comes the cheap physical layer that senior techs check before they open a laptop tree: fuel filter restriction, air filter restriction, a charge air cooler or boost leak, battery voltage and the grounds, oil level and whether the oil is where it should be, and a look at the harness for chafe. Three codes with two on sensors very often means one bad ground, one corroded connector, one rub-through, or a network problem rather than three dead sensors, and a candidate who says that out loud has done this work. Only then do they go to the OEM troubleshooting procedure and run pinpoint tests, which should sound like voltage drop under load at the connector or a breakout box at the ECM, not a static resistance reading on a circuit at rest. Finish means clearing codes, road testing loaded, and confirming the parameter actually moved. Be cautious with anybody who names a part in the first sentence, and be equally cautious with the answer that is just a tool: hooking up JPRO or the OEM software reads codes, it does not diagnose, and a tech whose entire method is reading a code and replacing what it names is the parts-changer this question exists to find.

A truck comes in with a DEF or SCR fault and a speed derate, and the driver says it has needed a parked regen every few days for two weeks. How do you work it? Where do you look for a cause upstream of the aftertreatment, and how do you decide whether a DPF gets cleaned or replaced?

What this assesses: Aftertreatment is most of a modern fleet's downtime and the place where weak techs come apart, so weight this question heavily. The tell of a real answer is that frequent regens get treated as a symptom of something making soot rather than as a filter problem. Strong answers go upstream: a leaking or unbalanced injector, an EGR cooler or valve, a turbo or VGT not doing what it is commanded to, a charge air leak, the wrong oil or fuel dilution, or a duty cycle of short runs and long idle that simply never gets the system hot enough, which is a dispatch conversation rather than a repair. On the DEF side, listen for checking fluid quality with a refractometer instead of eyeballing the tank, looking for contamination or the wrong fluid entirely, checking dosing pressure and the injector, and knowing that a NOx sensor can drift or an exhaust leak ahead of one can make it lie to the module. On the filter itself, the distinction that separates real experience from vocabulary is soot versus ash: soot burns off in a regen, ash does not and accumulates until the filter needs a cleaning service or replacement, so the decision comes from delta pressure read against soot load and service history, and from confirming the delta-pressure lines are not plugged before believing anything they report. The best answers also handle the truck as a business asset and mention warning dispatch before the derate stages down, since on most platforms the last step is a 5 mph speed limit and a stranded truck. One hard filter belongs here. If a candidate volunteers that they can delete emissions equipment or tune around a derate, score it as a no regardless of how strong the rest of the answer is. Tampering with emissions controls violates federal law, the EPA has brought enforcement against shops and individual technicians, and the same repair voids warranty coverage and fails at roadside. That is not a skills gap you can coach around.

How much PM and DOT annual inspection work have you done, and are you qualified to perform and sign an FMCSA annual inspection? Walk me through how you check brake adjustment and what you actually measure. Then take me through a wheel-end job: bearing adjustment, seal, and wheel torque.

What this assesses: This is the bread and butter a fleet shop runs on, and the answers come back in measurements or they come back in adjectives. On brakes, strong answers describe marking the pushrod and measuring applied stroke against the limit for that chamber size at full application pressure, not eyeballing the slack adjuster, plus lining thickness, drum or rotor condition, the automatic slack function, chamber and hose condition, and the air dryer and moisture in the tanks. Brake adjustment and brake system defects sit perennially at the top of the roadside out-of-service list, so a tech who cannot tell you what they measure is a compliance exposure and not just a slow hire. On wheel ends, listen for the bearing being set by procedure rather than by feel, the TMC adjustment sequence or the OEM equivalent, a new seal every time, the correct lubricant at the correct level, and wheel nuts torqued with a torque wrench to spec, generally in the 450 to 500 foot-pound range on hub-piloted disc wheels, followed by a retorque after the first 50 to 100 miles. Be cautious with any version of running them up with the impact and hitting them again later, because that is how wheels come off, and a wheel-off is the failure that ends a career and puts your DOT number under a light. On the inspection half, strong answers know whether they meet the inspector qualification and how it was documented, since the rule puts that burden on the carrier's file rather than on the technician's memory. Be cautious with a candidate who has done almost no PM, who is not qualified to sign inspections when the role requires it, or who treats PM as beneath them. PM throughput is what keeps the diagnostic bay from becoming the only bay, and a tech who resents it will be slow at it on purpose.

Show me how you write up a repair. Take a job you finished recently and give me the complaint, the cause, and the correction the way you would put it on the repair order. If it was a warranty job, how did you code the causal part and document the diagnostic time?

What this assesses: Most shops never ask this and most shops lose money to it. A warranty claim gets paid on the story, and a claim written as found code, replaced sensor gets denied or short-paid, at which point the shop eats parts and labor for work that was done correctly. The same goes for a customer dispute on a cash job, where the only thing standing between you and eating the ticket is what the tech wrote down. Strong answers separate the three cleanly and do not blur cause into correction: the complaint in the driver's words, the cause named as the failure with the test result that proved it rather than just a part number, and the correction including what they verified afterward. They mention the things that make a claim payable, which is usually the failed part kept and tagged, photos, readings before and after, and accurate mileage and hours. They code one causal part and can say why it was that one. Be cautious with a write-up that is a list of parts, with a tech who cannot say what proved the cause, and with anyone who treats paperwork as the service writer's problem, because that attitude has a direct dollar cost you will only see at the end of the quarter. There is a format point worth noticing here too. Asking a tech to write up a job in writing is not a proxy for the job, it is the job task itself, so the answer is a work sample rather than an interview performance. Some very good diesel techs are unimpressive on the phone and precise on paper, which is exactly the candidate a phone screen loses and a written screen finds.

Tell me about a repair that came back, a misdiagnosis, or a job a driver or customer was unhappy with. What was the actual root cause, what did the rework cost in hours or parts, and what did you change in how you work afterward?

What this assesses: Every tech with real hours has had a comeback, so a denial here is a bigger flag than an admission. What you want is a specific failure with a real bill attached: a sensor replaced when the problem was the circuit feeding it, a head gasket chased when the coolant was going out an EGR cooler, a regen forced on a filter that was full of ash, an air leak found on the third attempt, a bearing that came back because the seal surface was scored and nobody looked. Strong answers name the root cause correctly, put rough hours or dollars on the rework, and describe a concrete change in their own process afterward, which is usually a test they now run before condemning a part or a habit of confirming under load rather than at idle. The best answers include the version nobody has to admit to, which is a comeback they caught themselves before the truck left. Be cautious with a candidate who has never had one in fifteen years, with one who blames the part supplier, the driver, or the last shop without ever stating what was actually wrong, and with one whose only example is somebody else's mistake that they happened to find. In a flat-rate shop, comebacks are also an economic question rather than only a character question, so ask how their old shop handled comeback time. A tech who was never charged for rework and a tech who ate every hour of it learned very different habits, and you are hiring the habits.

This job means getting under trucks, working outside in the cold, lifting, and torch and grinder work, all of which you would do with or without a reasonable accommodation. What hand tools do you own and at what level, do you have reliable transportation, and can you be insured on a company service truck? Then: what do you do before you go under a raised truck or trailer, how do you handle a spring brake chamber, and what PPE do you actually use?

What this assesses: Four filters in one question, and the safety half is the one that puts somebody in a hospital when you skip it. The tool norm in this trade is that the technician owns hand tools and the shop supplies lifts, jacks, presses, specialty tooling, and the diagnostic software and subscriptions. A candidate who expects you to supply their wrenches has not held this job at level, and a heavy-duty tech typically has years of tool investment behind them, often with tool-truck payments still running, which is worth knowing because it is part of why techs leave over two dollars an hour. On safety, strong answers never go under anything supported only by a jack: stands rated for the load, cribbing, chocks, and support under the frame rather than the sheet metal on a trailer. Spring brake chambers store enough energy to kill somebody, so the answer needs to include caging the spring before removal and never cutting into a clamp-type chamber. Tires on multi-piece or split rims go in a cage. Strong answers know that a recent regen puts exhaust components well past 1000 degrees and they say what they check before reaching past one. They lock out stored hydraulic and electrical energy on vocational equipment, and if you run any hybrid or battery-electric units, they are honest about whether they have high-voltage training instead of assuming it transfers. PPE should sound like a default rather than an occasion: eye protection every time something gets cut or ground, gloves, hearing protection on air tools, and a respirator or at minimum wet methods around DPF ash, which is not something to breathe. Ask the physical and environmental part exactly as written, as demands of the job with the accommodation language, and stay out of health history entirely. On driving, ask about insurability rather than record in the abstract, because your carrier decides that one for you.

What shifts can you work, and are you open to nights, weekends, or on-call road service when a truck goes down away from the shop? Are you paid hourly or flat rate today, and what are you looking for? Any drug screen or background concern we should know about, and how soon could you start?

What this assesses: This is where qualified diesel hires fall apart, so surface it now rather than after the offer. Fleet shops run second and third shift for a structural reason, which is that trucks earn money during the day and get fixed at night, and that schedule is the most common reason a tech who cleared every other filter declines. Put the real shift in the posting and in this question and let people opt out while it is still free. Treat road service as its own willingness question and not a subset of overtime, because a tech who is fine with nights in a heated bay may want nothing to do with a shoulder on the interstate in February, and learning that at 2 a.m. is worse than learning it now. On pay, ask about structure and not only the number. Flat rate rewards a fast PM tech and punishes the diagnostic hour that a hard intermittent fault actually takes, which is why some of the best diagnosticians prefer hourly and some of the fastest producers will not leave flat rate. If you are changing somebody's comp model, explain it during the screen instead of on the offer call. On testing, if the role includes road-testing on public roads, that is usually a DOT-regulated function, which means a different program than your standard pre-employment panel; confirm it with whoever owns compliance rather than assuming. On start date, a working tech who offers to start tomorrow is either between jobs for a reason worth asking about or is leaving somebody the same way they will eventually leave you. Then score every applicant against the same [interview scorecard](/glossary/interview-scorecard) instead of reading answers one at a time, keep it a [structured interview](/glossary/structured-interview) so a dealer specialist and an independent generalist land on the same rubric, and run it as [asynchronous screening](/glossary/asynchronous-screening), because the tech you want is under a truck at two in the afternoon and answers at nine at night. In this trade [candidate drop-off](/glossary/candidate-drop-off) is the competitor you are actually losing to, and the shop that responds first usually gets the diagnostician.