Powertrain, generic (extended)

P20C3 Reductant Heater C Control Circuit Low

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

A low C circuit points to heater C being grounded somewhere or drawing excessive current. GM's 2025 DEF module treats heater 3, its second tank heater, exactly like heaters 1 and 2: a low-band voltage with the output off, or drain voltage and current limits with it on, Type A once 2 s have elapsed. On the 2014 LML Duramax (6.6 L) the glow plug control module reports heater currents above 25 A within about 1 s (Type B). The best-documented case is GM bulletin 22-NA-112: on 2020–2022 Silverado and Sierra with the 3.0 L LM2 or 6.6 L L5P, P20BB and P20C3 could set without any defect, and a recalibration of the DEF control module changed the test. Deutz maps the code to its return line heater (FMI 4). On those trucks software comes first; elsewhere look for a chafed harness.

Likely causes

Outdated DEF control module calibration on 2020–2022 GM diesel pickups (22-NA-112)

Heater C wire grounded through chafing

Shorted heater C element drawing overcurrent

Symptoms

Malfunction indicator lamp, possibly with no real fault present
Driver message that speed will be reduced without service
Unheated tank or line section in frost

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 c

References: static.nhtsa.gov, gsi.ext.gm.com, gsi.ext.gm.com, serdia.deutz.com.