Powertrain, generic (extended)

P20C0 Reductant Heater B Control Circuit High

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P20C0 reports that heater B's control circuit is high, typically a wire touching a battery feed or a driver that will not switch off. On Ford diesels B is the pressure line heater: with the power stage off, a current below 3 A while the output still carries over 5 V triggers it, and the glow plug module flags the condition in 250 ms. GM's 2025 tables test heater 2 (line heater) with the output off and an elevated voltage window, Type A with a two-second filter; the 2018 tables use a short-to-battery status from the DEF control module. VW uses more than 2.20 A reported by the power stage on the Touareg CNRB, and Deutz FMI 3 for its pressure line heater. A line heater that is powered all the time can overheat the hose, while one the module has shut down leaves it frozen.

Likely causes

Line heater control wire shorted to a positive feed

Heater output stage in the DEF, engine or glow plug module stuck on

Bridged pins in a shared connector next to a supply terminal

Symptoms

Malfunction indicator lamp and DEF warning
Line heater may stay on with the engine off, draining the battery
Frozen pressure line once the module disables the heater

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€60–700

Time0.5–3 h

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

Parts involved

reductant line heater b

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