Mexican Diesel Fuel/Sulfur Effects Update

Ed White wrote:
Hi almcc,

When the Bad DEF warning comes on while using ULSD, in Canada or the USA, it can be caused by a number of things, but the DEF (Diesel Exhaust Fluid) is almost never the culprit. My contacts in emissions systems programming have tested numerous samples of DEF sent back to them by dealerships and have never found a sample actually out of spec.

Typically a Bad DEF warning is initiated when the NOX sensor downstream of the SCR registers excessive NOX levels in the exhaust. The ECM will increase the dosing levels of DEF in the exhaust in an attempt to correct the situation, and if this fails, a fault code will occur. A temporarily blocked DEF injector can also cause a BAD DEF warning.

It is impossible to state with certainty what causes occasional self-correcting fault codes, like the one you described, but it could occur if, for example, the NOX sensor had become contaminated with deposits which are subsequently burned off during a regeneration cycle, or the temperature in the SCR temporarily dropped below the ideal for proper NOX reduction, or the NOX sensor is faulty, or exhaust by-products of additives placed in the fuel by the owner have caused fouling of the NOX sensors.

Thanks Ed!

Why I suspect solidified DEF is that the tank was running low and I hadn't refilled it (no indication) combined with few miles and mostly short runs on the truck following a winter in the warm south. The tank doesn't have a sophisticated level sensor system, it's kind of like the black tank sensors in our RV! The truck gets lots of miles in the winter but sits in the driveway most of the summer. Not good for the emission systems!