Powertrain, generic (extended)

P20BC Reductant Heater A Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P20BC means the heater A control circuit reads high: the wire or output has picked up battery voltage, so the element may run when it should be off or cannot be switched at all. GM's 2025 DEF tables spot this on heater 1 (tank heater 1) mostly in the off state, when the high-side voltage lies between roughly 2 and 4.3 V instead of near zero; Type A after a 2 s debounce. Ford's criterion is current below 3 A with more than 5 V at the output while the power stage is off. GM's 2014 tables for the 6.6 L LML, where the glow plug module drives the heaters, look for more than 3.5 V at the heater pin (Type B). On the Touareg CNRB, VW sets it above 2.20 A signal current. For 2025 Ram 2500–5500 with the 6.7 L Cummins, FCA names P20BC the suction line heater and fixes it with DCU software (08-076-26).

Likely causes

Heater A control wire shorted to a battery-voltage circuit
DEF control unit software fault (FCA 08-076-26 for 2025 Cummins Ram)
Heater output stage or relay stuck switched on

Symptoms

  • Check engine light and DEF system warning
  • Heater may stay energized, risking overheating or a drained battery
  • Reduced power or speed limit once the inducement starts

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€0–1,200
Time0.5–3 h

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

Parts involved

reductant line heater a

Related codes

P20B9P20BAP20BBP20C0P20C4P20C8P202C

References: gsi.ext.gm.com, static.nhtsa.gov, fordservicecontent.com, gsi.ext.gm.com.