Powertrain, generic (extended)

P20C8 Reductant Heater D Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

Diesel exhaust fluid freezes at about -11 °C, so SCR systems carry several electric heating elements in the tank, pump and lines; which one a maker calls heater D varies. P20C8 reports that the control circuit of heater D reads high, meaning the output or its wiring has contact with battery voltage and the element can no longer be switched as commanded. GM names P20C8 for the 3.0 L LM2 Duramax in bulletin 21-NA-149, where a chafed engine harness blows fuses and sets a long list of codes. GM's 2025 DEF tables test the comparable heater C circuit (P20C4) through drain voltage and load current and rate it Type A after 2 s. Open, low and performance faults of heater D are P20C5 to P20C7. In frost the fluid may stay solid, dosing stops and the SCR warning follows.

Likely causes

Chafed harness letting the heater D control wire touch a battery-voltage feed
Moisture or DEF residue in the heater connector bridging pins
Internal short inside heater D or its integrated pump or line assembly
Faulty heater output stage in the engine or DEF control module

Symptoms

  • Check engine light and DEF or emission system warning
  • DEF dosing unavailable in cold weather, NOx warning or later speed limitation
  • Blown fuses and further unrelated codes when a chafed harness is the cause

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–900
Time0.5–3 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant line heater d

Related codes

P20C5P20C6P20C7P20C4P20C0P20BC

References: static.nhtsa.gov, gsi.ext.gm.com, law.cornell.edu.