Powertrain, generic (extended)

P20BB Reductant Heater A Control Circuit Low

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

A low reading on the heater A control circuit usually means the output or its wire is grounded, or the element pulls far more current than it should. GM's 2025 DEF module checks heater 1 (tank heater 1) in both states: switched on, a low-side drain voltage above 0.45 V or a load current above 95 A counts; switched off, the high-side voltage sitting in a low band near 0.5–0.6 V. Two seconds of failures give a Type A fault. Ford logs P20BB on circuit 1 above 15 A and lets the glow plug module report it within 250 ms; VW uses a signal voltage under 2.97 V on the 2014 Touareg. On 2020–2022 Silverado and Sierra diesels GM found the test itself setting P20BB falsely and released new DEF module calibrations (22-NA-112). Expect a warning lamp and, later, a speed limit.

Likely causes

Heater A wire chafed through and touching ground

Internal short in the heater element, drawing excessive current

DEF control module calibration setting the code falsely (GM 22-NA-112)

Moisture bridging heater pins to ground in the connector

Symptoms

Malfunction indicator lamp, possibly with a blown heater fuse
Message that speed will be reduced if the system is not serviced
No DEF heating, so dosing is delayed in frost

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€0–1,500

Time0.5–4 h

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

Parts involved

reductant line heater a

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