OBD-II code · Powertrain (generic, extended)
P20B5 Reductant Metering Unit Heater Control Circuit/Open
The control circuit for the heater inside the DEF metering unit is open — winter operation cannot keep the dosing block above freezing.
Is it safe to drive with P20B5?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyHeater winding open
- Less likelyPin pushed back out of connector body
- Less likelyCracked solder joint inside the module or sensor
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Repair at a glance
- Difficulty
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1–4 h
- Emissions test
- Likely to fail
- Parts involved
- Reductant Metering Unit Heater
Related codes
- P20B2Reductant Heater Coolant Control Valve Performance/Stuck Open
- P20B3Reductant Heater Coolant Control Valve Circuit Low
- P20B4Reductant Heater Coolant Control Valve Circuit High
- P20B6Reductant Metering Unit Heater Control Circuit Performance
- P20B7Reductant Metering Unit Heater Control Circuit Low
- P20B8Reductant Metering Unit Heater Control Circuit High
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.