Powertrain, generic (extended)

P20B9 Reductant Heater A Control Circuit/Open

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

P20B9 flags an open or broken control circuit for reductant heater A, one of the electric elements that keep diesel exhaust fluid liquid in winter. Which element carries the letter A differs by maker: GM's 2025 DEF tables call it heater 1, the tank heater; Ford links the code to the pump/heater assembly in the tank, Deutz to the suction line heater. A Hino bulletin for 2015–2018 J05 cab-over trucks instead lists P20B9 as heater 2 of the heated DEF line, about 5.5 ohms per circuit at 20 °C. GM pinpoints the open with the output switched off and stores it after 2 s as Type A; VW sets it on the 2014 Touareg CNRB below 4.70 V signal voltage. Ford's TSB 21-2336 blames water inside reductant harness connectors on 3.0 L and 6.7 L trucks. Without heat, dosing waits until the fluid thaws.

Likely causes

Water or DEF residue inside the heater or tank harness connectors (Ford TSB 21-2336)

Heater A element burnt out, so resistance is far above specification

Broken supply or return wire between control module and heater

Heater output stage in the DEF or engine control module defective

Symptoms

Check engine light, often first noticed in cold weather
DEF dosing starts late or not at all after a cold start
DEF system warning followed by a countdown to a speed limit

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€80–1,500

Time0.7–4 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, static.nhtsa.gov, static.nhtsa.gov.