Cummins Fault Code
Free · no signupSPN 157 FMI 18
Derate (5 mph / 8 kph). The truck is restricted to "crawl" speed.
System
Emissions
Symptom
Derate (5 mph / 8 kph). The truck is restricted to "crawl" speed.
Diagnosis & fix steps
DEF Pressure Too Low.
LOGIC: The aftertreatment system cannot build the required 5 bar (approx 72 psi) of DEF pressure.
ACTION: Check for frozen DEF lines in winter or a clogged DEF pump filter.
Red Seal field insights
Pro-Tip: If the pump is humming but there's no pressure, the "inlet strainer" at the bottom of the DEF tank is likely crystallized. Wash it with hot distilled water to dissolve the urea crystals.
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