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Diesel truck guides · Licensed & Insured · Maryland · 24/7 dispatch

DEF Light On or Truck Derated: What to Do

When the DEF light comes on, treat it as a warning and act early: check your DEF level, top it off with proper diesel exhaust fluid if it is low, and note any other lamps or codes. If the light stays on after refilling, or the truck is already derated, the cause is likely in the DEF or SCR system, such as a sensor, pump, heater, or fluid quality problem. Call (240) 780-8691 and a mobile diesel mechanic can diagnose it, 24/7.

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What the DEF light is telling you

Diesel exhaust fluid (DEF) is injected into the exhaust of modern diesel engines by the selective catalytic reduction (SCR) system to reduce NOx emissions. The truck monitors DEF level, quality, and whether the SCR system is working. When it sees a problem, it lights a warning.

If the problem is not corrected, the engine computer can escalate the warnings and eventually limit engine power or vehicle speed. How and when that happens varies by engine maker and model year, so follow what your dash tells you and do not wait.

Common causes of a DEF warning or derate

  • Low DEF level. The simplest cause and the first thing to check.
  • Wrong or poor-quality fluid. Contaminated DEF, diluted DEF, or the wrong fluid in the tank can trigger a fault.
  • DEF pump, injector (doser), or line problems. A failed pump, a clogged doser, or a damaged line can stop proper dosing.
  • Heater or temperature faults. DEF can freeze in cold weather, and the system uses heaters to keep it usable. A heater or temperature sensor fault can set a warning.
  • Sensor and wiring faults. Level, quality, and NOx sensors, or damaged wiring, can set faults even when the fluid is fine.
  • SCR efficiency faults. The system sees that NOx is not being reduced as expected, which needs diagnosis to trace.

What to do right now

  • Check the DEF level and top off with DEF that meets the standard for your system. Do not use water or any other substitute.
  • Never put DEF in the fuel tank, and never put diesel fuel in the DEF tank. If that happens, do not start the engine and call us.
  • Write down every lamp and message on the dash, and any codes you can read.
  • If the light stays on after refilling, or the truck is derated, have the system diagnosed rather than clearing the warning and hoping.
  • If the truck is derated on a roadway, get to the safest spot you can, turn on hazards, and see our semi truck breakdown guide.
  • Do not bypass, disable, or defeat the SCR system. That creates bigger problems for the truck and the fleet.

How we repair DEF and SCR faults

Our mobile diesel mechanic diagnoses DEF and SCR faults at the roadside, at your yard, or at a job site. We find the failed part or fluid issue, repair it, and clear the derate when the truck can be made safe and legal on site. We repair and restore proper emissions systems; we do not do emissions deletes or delete tuning.

More on the service: DPF, DEF and derate repair and mobile diesel mechanic. If you also want to understand the term, read what a DPF derate means.

DEF light and derate FAQ

What does the DEF light mean?

It warns of a problem with the diesel exhaust fluid or the SCR system, such as low fluid, poor fluid quality, or a component fault. Check the level first, and have it diagnosed if the light stays on.

Will my truck derate if I ignore the DEF warning?

If the problem is not corrected, the engine computer can escalate its warnings and then limit power or speed. The timing varies by engine maker and model year.

Can I just top off the DEF and keep driving?

If the level was low and the warning clears, that may be all it was. If the light stays on or the truck is derated, get it diagnosed.

What if I put DEF in the fuel tank?

Do not start the engine. Call (240) 780-8691 so a mobile diesel mechanic can advise and help.

Will you disable the DEF system to stop the light?

No. We repair and restore proper emissions systems. We do not do emissions deletes or delete tuning.

Do you work on cars or pickups?

No. We service commercial diesel trucks only, not passenger-car auto repair.

Mobile diesel service from four Maryland locations

Mobile service rolls from the closest Priority location to the roadside, your yard, or a job site. See the full service area. Shop diesel repair is available only at the Randallstown location.

Related: DPF derate repair · mobile diesel mechanic · truck road service · contact / fleet quote · service area · all diesel truck guides

DEF light or derate? Call dispatch · Maryland

Tell dispatch the truck, your location, and what the dash shows. A mobile diesel mechanic can diagnose it roadside or at your yard, 24/7.

Call (240) 780-8691 Request Fleet Quote DPF derate repair