Powertrain, generic (extended)

P20BF Reductant Heater B Control Circuit Low

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

For heater B the code stands for a grounded control circuit or an overcurrent. Ford, where B is the pressure line heater, sets it when circuit 2 draws more than 15 A with the power stage on; the glow plug module can report such a short within 250 ms. GM's 2025 DEF module applies to heater 2 (line heater) the same low-band and overcurrent tests it uses on the tank heater, Type A, confirmed within 2 s, and in 2018 relied on a short-to-ground status from the DEF control module. VW's Touareg CNRB stores P20BF below 2.97 V signal voltage; Deutz lists FMI 4, a voltage too low, for its pressure line heater. Chafed line harnesses along the frame and water in connectors are typical. Because the driver shuts down or a fuse fails, the pressure line stays unheated in winter.

Likely causes

Line heater wire chafed on the frame and grounded

Water in the line heater connector creating a path to ground

Internal short in the heated pressure line

Symptoms

Check engine light, sometimes with a blown fuse
DEF pressure fault codes in cold weather
Speed limitation if the fault persists

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