P20B9: Reductant Heater "A" Control Circuit/Open
The AdBlue (DEF) heater circuit has a fault.
Normal risk
Usually drivable. Dosing may fail in cold weather.
Short answer · Can I keep driving with P20B9?
Usually yes. In cold weather the AdBlue may not thaw, leading to further warnings. Have it checked soon.
What does P20B9 mean?
Code P20B9 means the reductant heater A control circuit is open or faulty.
The heater thaws AdBlue in cold weather. An open circuit or failed element means dosing may not start after a cold night.
What the car does when the code is set
The AdBlue warning or check engine light comes on. In mild weather the system usually works; in frost the AdBlue may not thaw, which then leads to further warnings and a restart countdown.
How the system works
AdBlue freezes at about −11 °C. To keep the SCR system working in winter, the tank, pump and lines have electric heaters that thaw the fluid after a cold start and keep it liquid. The engine control unit or a separate SCR control unit switches these heaters and monitors their current. If a heater circuit is open or shorted, it stores a heater code; in mild weather the system usually still works.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Faulty heater elementThe heater in the tank or line fails.
- Wiring or connector faultCorrosion from AdBlue crystals.
- Fuse or relay faultThe heater supply fails.
How it is diagnosed
- Check fuse and relayFollow the wiring diagram.
- Check the connectorLook for white crystals and corrosion.
- Measure the heaterCompare resistance with the service data.
- Repair and clearHeaters are often part of the tank unit.
Questions about P20B9
Can I still drive in winter with P20B9?
You can drive, but in frost the AdBlue may stay frozen. The system then cannot dose, and a restart countdown may start. Have the heater repaired before cold weather.
Is the heater part of the AdBlue tank?
On many cars the heater is built into the tank or pump unit and is replaced with it. Some line heaters can be replaced separately.
What do white crystals on the connector mean?
AdBlue leaves white urea crystals where it leaks or creeps. Crystals on a connector can corrode it and cause heater circuit faults.
Why does AdBlue need a heater?
AdBlue freezes at about −11 °C. The heater thaws it so dosing can start in cold weather.
Will the warning go away in summer?
The heater fault stays stored. Have it repaired before winter.
Does a heater fault use more AdBlue?
No, consumption depends on driving, not the heater.