P20B9Reductant Heater A Control Circuit/Open
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- 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.
- What you may notice
- 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
- Likely causes
- LikelyWater or DEF residue inside the heater or tank harness connectors (Ford TSB 21-2336)
- PossibleHeater A element burnt out, so resistance is far above specification
- PossibleBroken supply or return wire between control module and heater
- RareHeater output stage in the DEF or engine control module defective
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €1,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.