The truck is running fine, and it will not go faster than 5 mph. The engine is not broken. It is being held back on purpose, by its own computer, because the emissions system has decided the truck should not be on the road.
This is what a 5 mph derate is, what caused it, what the truck was telling you before it happened, and what to do in the next 20 minutes.
What the derate is
Nearly every diesel truck built for the US market since 2010 has selective catalytic reduction: a DEF tank, a dosing injector, an SCR catalyst, and NOx sensors before and after it. Federal rules require the engine to enforce the system. If the DEF runs out, the DEF is wrong, or the SCR system has a fault the engine cannot verify is fixed, the ECM steps through an inducement ladder.
The ladder is roughly the same on every make, with different timing:
- A DEF or SCR warning lamp, usually with a message on the dash.
- A torque derate, often 25 percent, after a set time or distance.
- A speed limit, 55 mph on some makes, and then 5 mph after a further countdown, or at the next refuel or key cycle.
The 5 mph stage is called the final inducement. The engine will still run. It will move the truck off the road. It will not haul a load.
Separately, a DPF that is plugged can trigger its own derate, and some engine faults (overheat, low oil pressure, a critical sensor) derate for protection. The dash message tells you which family you are in. Read it, and write it down.
What triggers it most often
In the order we see them at 820 Matzinger Rd:
- DEF level. The tank ran empty, or the level sensor says it did. The simplest one.
- DEF quality. Water, diesel or the wrong fluid in the tank; DEF that is old or was stored hot; a quality sensor that has failed. The ECM sees the concentration out of range and starts the clock.
- SCR efficiency. The outlet NOx sensor sees too much NOx getting through. Causes: a dosing injector that is crystallized and not spraying, a DEF pump not building pressure, a DEF line heater that failed so the fluid froze, a catalyst that is contaminated or failing, or a NOx sensor itself reading wrong.
- Frozen DEF. January in Toledo. DEF freezes at 12 degrees Fahrenheit. The tank and lines have heaters; when a heater fails the truck cannot dose, and after a grace period it derates.
- DPF. Soot load too high, usually after weeks of failed or interrupted regens. The DPF lamp and the high exhaust temperature lamp were on before this.
- Tampering detection. If anything in the aftertreatment has been unplugged or modified, the ECM derates and logs it.
What the dash was telling you before
A 5 mph derate is never the first message. There was a lamp, then a countdown, then a torque reduction. Common fault codes to look for in the memory, by SPN:
- SPN 1761: DEF tank level.
- SPN 3364: DEF quality.
- SPN 4364: SCR conversion efficiency.
- SPN 3226: outlet NOx sensor.
- SPN 3216: inlet NOx sensor.
- SPN 3251: DPF differential pressure.
The SPN numbers are standard across makes. The dash may show a code number of its own; the shop reads the SPN and FMI from the ECM and works from those.
The next 20 minutes
Minute 0 to 2: get off the road
At 5 mph you have enough to reach the shoulder, a ramp or a lot. Get fully out of the travel lane. Hazards on. If you are stopped on the shoulder, triangles at 10, 100 and 200 feet behind the trailer, per 49 CFR 392.22. Do not try to limp it to the shop on the interstate at 5 mph.
Minute 2 to 5: read the dash, and do not shut it off yet
Write down every message and every lamp. Note the DEF gauge. Take a photo of the dash. If the truck has a driver display with a fault menu, scroll it and write down the codes. Some faults are easier to diagnose while the condition is live; once the key cycles, some of that data goes with it.
Minute 5 to 10: check the obvious
- DEF level. If the gauge is empty or near it, that is probably the fault.
- DEF cap and tank. Does the fluid look right? DEF is clear and slightly thick. Cloudy, oily, or smelling of diesel means contamination.
- Winter: is the DEF frozen? A tank that is a solid block in a truck that sat all night is a heater that did not run.
- Was anything done to the truck recently? A sensor unplugged, a wire pinched during a tire change, a repair that disturbed the aftertreatment harness.
Minute 10 to 15: if it is DEF level, fill it
If the tank is empty, add DEF. Real DEF, from a sealed container: not water, not diesel, not whatever is in the jug behind the seat. On most makes, filling the tank and cycling the key clears the level inducement once the ECM sees the new level; some require the truck to run or drive a short distance first. It will not clear a quality or efficiency fault.
If the tank is full and the truck says it is empty, the level sensor or the DEF head has failed. That is a shop job.
Minute 15 to 20: call, and tell us what you wrote down
Call (419) 724-2475 if it is between 8 and 5 on a weekday. Tell us the make and year, the dash messages, the codes if you have them, the DEF level, and where you are. We will tell you one of three things:
- Fill the DEF and try a key cycle; you are probably fine.
- Bring it in. It will move at 5 mph if you are close, or it needs a tow if you are not.
- The service truck is coming to you, during shop hours, in the Toledo area.
What not to do
- Do not disconnect the batteries to reset it. The fault is stored. On most makes you have now added a tampering event and lost the data the tech needs.
- Do not add anything to the DEF tank but DEF.
- Do not keep driving on a torque derate hoping it clears. The countdown is running.
- Do not buy a delete. It is a federal violation, it voids the warranty, it fails the CARB Clean Truck Check if you run California, and it gets found at an inspection.
How the shop clears it
The derate does not clear because a part was replaced. It clears when the ECM verifies the system is working: a DEF quality reading in range, an SCR efficiency test passed, a soot load back under the threshold after a forced regen. The tech reads the codes, tests DEF pressure and dosing, checks the NOx sensors, runs the OEM diagnostic routine, repairs what failed, and then runs the verification so the inducement resets. On some makes a one-time inducement override can move the truck to the shop; it is not a fix, and it does not come back a second time.
We are Cummins certified and International certified, and we do DPF cleaning and forced regens in the shop. A DEF level derate is usually done the same day. A DEF pump, dosing injector or NOx sensor depends on parts on the shelf; we tell you before we start.
Before it happens next time
- Keep the DEF tank above a quarter. Fill it when you fuel.
- Buy DEF from a high-turnover source, and do not leave an open jug in the sun.
- In winter, plug the truck in, and let the DEF heaters run before the derate countdown does.
- Watch the regen count. A DPF regenerating more often than it used to is a problem upstream: an injector, a turbo, an EGR cooler.
- Put the aftertreatment on the PM. We pull the regen history and the DEF system data every service, and a dosing injector that is starting to crystallize gets cleaned before it dies.
The truck told you. Next time, it gets heard on the PM instead of on the shoulder.