Paccar MX-13 Derate and SCR Efficiency Low - Possible DEF Crystal Build Up

Question

My 2018 Peterbilt 579 is stuck in a 5 mph derate and showing an SCR efficiency low warning on the dash. I can clear the codes with my laptop but they come back as soon as the exhaust hits operating temperature. I am worried the decomposition pipe is restricted or the doser is failing.

Sam Holloway · Accepted Answer

Found the issue was a completely cracked and leaking DEF Dosification Valve (Part #2115825) and a crystallization 'volcano' in the decomp pipe. I replaced the doser valve, removed the decomp pipe to pressure wash the urea buildup out, and performed a 'Reductant System Self-Test' and 'SCR Conversion Test' using DAVIE4. Once the tests passed, the derate cleared automatically without needing a forced regen. Total cost for the valve was about $450 and 3 hours of labor.

Chris Donovan · Answer

Check your DEF pump pressure in the diagnostic software. If you aren't maintaining at least 5 bar steady, you might have a failing pump or a restricted filter in the DEF tank supply line.

shoptech_carlos · Answer

Check the coolant lines going to the DEF doser. If the valve is stuck open and overheating the DEF before it sprays, it causes poor atomization. Also, perform a DEF quantity test to ensure the doser isn't partially clogged.

Lina Brooks · Answer

Take a look at the decomposition pipe where the DEF injector sprays. If there is a massive white crystal buildup (the 'volcano'), the DEF won't vaporize correctly and the SCR catalyst can't reduce the NOx levels, triggering the derate.

Paccar MX-13 Derate and SCR Efficiency Low - Possible DEF Crystal Build Up

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