What it means
Metering unit heater is electrically OK but does not warm the block within the expected time — partial winding fault or excessive heat loss.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant metering unit heater warms the dosing block to keep DEF usable in cold weather. P20B6 means the heater circuit is present but the unit does not warm as quickly or sufficiently as the module expects, which can result from a weak heater, poor connections, wiring damage, or excess heat loss.
Common causes
Most likely first.
- Metering unit heater partially failed
- Corroded or loose heater connections
- Damaged heater wiring
- Excessive heat loss from the metering unit
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)
How to diagnose it
- Save freeze-frame data and compare the metering unit temperature trend with the heater command and ambient conditions.
- Inspect the heater connector and wiring for looseness, corrosion, damage, or poor routing that could reduce power delivery.
- Use a scan tool's supported heater command and monitor whether the metering unit temperature rises as expected.
- With the circuit isolated as specified by the manufacturer, test heater resistance and supply wiring against the manufacturer's specification.
- If electrical checks pass, inspect the metering unit and its surroundings for damage or conditions that could cause excessive heat loss.
Common fixes
- Secure loose heater connections and clean corroded terminals.
- Repair damaged heater wiring or connectors.
- Replace a weak metering unit heater or assembly confirmed by testing.
- Correct damaged insulation or mounting that allows excessive heat loss.
Can I keep driving?
It may be drivable in warmer weather, but slow heating can leave DEF unable to dose properly in cold conditions and may lead to emissions warnings or a no-start countdown. Avoid relying on the vehicle in freezing temperatures until heating performance is restored.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.