An engine fault is the one repair where guessing is most expensive. A code is a symptom, not a diagnosis, and the difference between reading it and understanding it is usually several thousand dollars.
Start with what the code actually means
Modern diesels throw fault codes for conditions rather than for parts. A sensor code very often means the sensor is reporting something real, and replacing it makes the light go away without fixing anything. We read the freeze-frame data, look at what the engine was doing when the fault set, and test the circuit before anything gets ordered.
That matters most with derates. A truck that has dropped to 55mph, or to 5mph, is telling you about a chain of conditions — usually aftertreatment, sometimes fuel or air — and clearing the code without resolving the chain just buys you the distance to the next shutdown.
The engines we see
- Detroit DD13 and DD15 — EGR coolers, one-box aftertreatment faults, turbo actuators and injector harness problems.
- Paccar MX-13 in Kenworth and Peterbilt — aftertreatment and DEF dosing, EGR valves, and coolant issues that present as something else entirely.
- Cummins ISX and X15 — injectors, EGR, turbo actuators, and the familiar cam and lash-adjuster maintenance that gets skipped.
- Volvo D13 — injector cup failures that put coolant where it must not be, EGR and aftertreatment work.
- International A26 and the legacy MaxxForce engines, which need different expectations set before work starts.
- Mack MP7 and MP8 — aftertreatment, EGR and injector work.
Aftertreatment is most of modern engine work
DPF, DEF and SCR problems are the single largest category of engine complaint we take, and most of them are not really faults — they are duty cycle. A truck doing short local runs with heavy idle never gets hot enough to regenerate properly, soot loads, and the system starts reporting. Replacing a DPF on that truck fixes it until it does not.
We would rather tell you that, and talk about forced regens on a schedule, idle policy and route patterns, than sell you the same part twice. More on DPF, DEF and SCR work.
What can be done on the road and what cannot
Sensors, harnesses, connectors, charging faults, air and fuel restrictions, leaking fittings and a fair number of derate conditions get resolved where the truck sits — that is mobile work. Head gaskets, injector cups, in-frame overhauls, turbo replacement and anything needing the engine opened belong in the bay at 1616 E 26th St with a lift and time.
If the truck will still drive safely, driving it in is almost always cheaper than a tow plus a road repair. If it will not, do not force it — a derated engine pushed to finish a run is how a fault becomes an overhaul.
Before you authorise an overhaul
An in-frame is a large decision and it deserves real numbers: oil analysis, blow-by, cylinder condition, and an honest view of the rest of the truck. We will give you that picture rather than a quote, because the right answer is sometimes that this chassis is not worth the engine.
Can you diagnose without replacing parts first?
That is the whole idea. Reading a code takes a minute; understanding it takes testing. We charge for the diagnosis and then you decide, rather than paying for parts that turn out to be a guess.
My truck is derated. Can I drive it to you?
Call first and describe the warning and the speed limit it has imposed. Some derates are safe to drive in on; some mean stopping now. We will tell you which, and the answer does not depend on whether we get the job.
Do you work on all makes?
Yes — Freightliner, Kenworth, Peterbilt, Volvo, International, Mack and Western Star, plus the equipment engines that come with yard trucks and forklifts. See our Tacoma truck shop for everything beyond the engine.
Related services
Everything below is available on the road where it can be, and in the Tacoma bay where it needs a lift. Same techs either way.
Truck down? Let's get you rolling.
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Book a repair →Call (425) 900-6212