DEF and SCR problems: the emissions faults that strand medium-duty trucks
The DPF gets most of the attention, but the DEF and SCR side strands just as many trucks. When the system that cleans up your exhaust throws a fault, the truck starts a countdown and then derates, and you are stuck. Here is what goes wrong and why.
What DEF and SCR do
Selective catalytic reduction, the SCR, sprays diesel exhaust fluid into the exhaust to break down the nitrogen oxides the engine makes. DEF is that fluid. The system constantly checks fluid quality, dosing, and the result with sensors, and if any of it is out of range, it flags a fault. The truck treats a tampered or non-working SCR system seriously, which is why these faults come with a derate.
The faults we see most
Bad DEF is a big one. Fluid that is contaminated, watered down, or has sat too long reads as poor quality and sets a code. DEF also crystallizes into a white crust around injectors and lines, which clogs dosing. NOx sensors fail and report numbers that do not add up. Pumps, dosing injectors, and the lines that feed them wear out or clog. And in a Colorado winter, DEF heater faults show up, because DEF freezes and the system has to thaw it before it can dose.
Why you cannot just ignore it
The system is required, and the derate is the truck forcing the issue. Run low on DEF or let a fault sit and the truck steps the power down until it is barely moving. Topping off the tank does not fix a sensor or a crystallized injector, so the countdown comes back.
What we do
We read the codes and the live data, find whether it is fluid, a sensor, dosing, or a heater, and repair the actual fault. We do not delete the system. We make it work so the truck is legal and out of limp mode. That is the exhaust and emissions work we do every week.
Keep it from happening
Use good DEF, keep the tank from running dry, and address the first fault instead of the third. A truck on a preventive maintenance schedule gets the early warning, and our engine and drivetrain diagnostics catch the problems that start upstream. Call the shop at 720.312.7095.