Liebherr Fault Code
Free · no signupP204C-00
DEF Derate. Check Engine light on; AdBlue/DEF pressure fails to build; machine may limit RPM/torque.
System
Aftertreatment (SCR)
Symptom
DEF Derate. Check Engine light on; AdBlue/DEF pressure fails to build; machine may limit RPM/torque.
Diagnosis & fix steps
Reductant (DEF) Pressure Sensor Circuit Low.
LOGIC: The SCR control module detects a voltage signal from the DEF pressure sensor that is below the minimum threshold (typically < 0.25V).
ACTION: Check the DEF pump harness for chafing. Verify 5V reference at the sensor.
Red Seal field insights
Pro-Tip: On Liebherr rigs, the DEF lines are heated. P204C-00 often occurs in cold weather if the DEF has frozen in the pump head and "cracked" the internal ceramic pressure sensor.
Red Seal Fix: Before replacing the pump, check the "Return Valve" (Purge Valve). If it's stuck open, the system can't build pressure, and the sensor correctly reports "Low," which the computer sometimes misinterprets as a circuit fault.
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