The DEF head sensor failed giving a warning that the engine would be derated if the engine was not shut off. The safety hazard with a derated engine is that 5 mph is top speed, not only endangering my vehicle, plus other vehicles near by. The head sensor was replaced by Cummins after purchasing a replacement tank with a new head sensor attached. The purchase was made from Spartan, the manufacturer of the Chasis. Individual head sensors were not available due to a nationwide shortage. I was first made aware of a...
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problem when a DEF warning buzzard and a check engine light started while driving down the interstate. I checked the DEF tank and found it to be 3/4 full and no leaks or noticeable problems. After Cummins replaced the tank with a new head sensor I was able to drive 2000 miles without another problem.