Detroit Diesel Fault Code
Free · no signupSPN 4334 FMI 4
Active 5 MPH Derate / DEF Lamp Solid
System
Emissions / DEF
Symptom
Active 5 MPH Derate / DEF Lamp Solid
Diagnosis & fix steps
LOGIC: DEF Line Pressure Sensor Circuit Failed Low (Short to Ground).
ACTION: Measure DEF pressure sensor signal voltage at the ACM pinout. Inspect harness routing near the transmission crossmember for chafing against the frame rail.
Red Seal field insights
Inducement Logic: Grounded signal simulates 0 bar DEF delivery line pressure, prompting the ACM to shut down the DEF pump to protect against dry-run cavitation.
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