Mobile Diesel Mechanic: Truck Repair at the Roadside or Yard

What does a mobile diesel mechanic work on?

A mobile diesel mechanic brings diesel tools, scan software and parts to your truck, whether it’s parked at home, at a jobsite, in a yard or broken down on the road. The work usually falls into three groups, and not every mechanic covers all three.

• Diesel pickups. Light-duty diesel trucks used for work, towing or daily driving. Many mobile mechanics who work on cars also handle these for everyday jobs like batteries, brakes and belts, but emissions and injection work often needs a diesel specialist.
• Box trucks and medium-duty trucks. Delivery, moving and service trucks. These may use hydraulic or air brakes, bigger batteries and heavier parts than a pickup.
• Semis and heavy-duty trucks. Tractors and trailers need heavy-duty tools, air brake knowledge and manufacturer-specific diagnostic software. Mechanics who work on these usually run a dedicated service truck.

When you contact someone, name the vehicle class and engine up front. A mechanic who is great with diesel pickups may not take a call for a tractor-trailer, and the reverse is also true. Our general guide to what a mobile mechanic can do covers the limits that apply to all mobile work.

Roadside breakdown repair or yard service: which do you need?

Mobile diesel work splits into emergency calls and planned work. The price, speed and what can be fixed on the spot differ.

A mobile mechanic can tell you if the job is realistic on site. If the truck needs a heavy tow to a shop, it is better to know early. See roadside assistance vs mobile mechanic for how emergency help usually works.

What are DPF and DEF, and why do they cause breakdowns?

Modern diesel engines use exhaust systems to cut pollution. Two parts cause many of the warning lights diesel owners see.

DPF: the diesel particulate filter

The DPF traps soot from the exhaust. Over time it fills up and the system burns the soot off, a process called regeneration. Trucks that do a lot of short trips or idling may not get hot enough to finish a regeneration, and the filter can clog. A clogged filter can trigger warning lights and reduced power, sometimes called limp mode. A mechanic may be able to start a forced regeneration with a scan tool, clean the filter or replace it, depending on its condition.

DEF: diesel exhaust fluid

DEF feeds the selective catalytic reduction (SCR) system, which turns nitrogen oxides into nitrogen and water. The Engine Technology Forum describes DEF as a non-toxic fluid of automotive-grade urea and purified water, stored in its own tank with a blue cap. It says DEF use averages about 3% of fuel use, and that on pickups the fill port is usually near the fuel filler, while heavy trucks carry the tank next to the diesel tank.

Running out of DEF, using contaminated fluid, or a failed DEF sensor, heater or pump can bring on warnings and power limits. Check your owner’s manual for how your truck warns you and what it does when DEF runs low. Never put DEF in the fuel tank, or diesel in the DEF tank. If that happens, don’t start the engine and call a mechanic.

These systems vary a lot by maker and model year. A mechanic who can see the truck and read the codes will know more than any guide.

What do the DOT annual inspection rules say?

If your truck is a commercial motor vehicle under federal rules, it needs a periodic inspection, often called the annual DOT inspection. The Federal Motor Carrier Safety Administration’s rules define a commercial motor vehicle to include one used on a highway in interstate commerce with a gross vehicle weight rating or gross combination weight rating of 10,001 pounds or more, along with some passenger and hazardous materials vehicles (49 CFR 390.5).

• How often: each component on the federal list must have passed an inspection at least once in the preceding 12 months, with documentation of that inspection on the vehicle (49 CFR 396.17).
• Who can inspect: the inspector must understand the inspection standards and be able to identify defective parts, and must qualify through a federal or state training program or state certification, or through at least one year of training and experience combined, such as work as a mechanic in motor carrier maintenance (49 CFR 396.19).
• Records: the inspection report must be kept for 14 months from the inspection date, where the vehicle is housed or maintained (49 CFR 396.21).

Some mobile diesel mechanics are qualified to do these inspections at your yard. Ask for proof that they meet the inspector qualification rules, and keep a copy, because the carrier is responsible for showing it if officials ask. State rules and intrastate operations can add their own requirements, so check with your state as well. This is general information, not legal advice. For state car inspections, see our glossary entry on safety inspections.

What should you do if your truck breaks down on the road?

For commercial trucks, federal rules require the driver to turn on the hazard flashers right away when stopped on the road or shoulder, and to place warning devices as soon as possible and within 10 minutes. On a two-lane road, that means one about 10 feet from the truck toward approaching traffic, one about 100 feet behind and one about 100 feet ahead (49 CFR 392.22). Rules differ for divided highways, hills and curves, so learn them for your situation.

Then gather the information below before you call for help.

What should you have ready when you call?

• Exact location: road, mile marker or exit, direction of travel, and whether you are on the shoulder or in a lot.
• Year, make, model, engine and VIN, plus the unit number for fleet trucks.
• What happened: warning lights, check engine light or DPF/DEF messages, sounds, smells, smoke, air leaks.
• Any fault codes shown on the dash or from your telematics system.
• Whether the truck is loaded, and with what, especially if it’s hazmat or refrigerated.
• Who authorizes the repair and how the bill will be paid.
• Recent work and service history if you have it.

Our guide to preparing for a mobile mechanic covers the basics for any visit.

What should you ask a mobile diesel mechanic?

• Which vehicle classes and engines do you work on?
• Do you have dealer-level diagnostic software for my engine and emissions system?
• What is your call-out or trip fee, your labor rate, and do after-hours or roadside calls cost more?
• Do you carry common parts, or will you need to source them?
• Can you perform annual inspections, and can you show your inspector qualifications?
• Will I get a written estimate before work starts and a written record of what was done?

We don’t have a sourced price range for diesel and heavy truck work, and it varies widely by truck class, parts and location, so get a written quote each time. Prices for car and light-truck work are on our mobile mechanic cost page.

What to do next

Search our directory of mobile mechanics by city and check whether listings cover diesel and truck work, or send a request with your vehicle class, location and the problem. If you run several trucks, our guide to mobile fleet maintenance covers scheduled service at your yard.