What it means
This code indicates a high voltage condition in the reductant metering unit temperature sensor circuit.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The temperature sensor in the DEF metering unit sends a signal used by the control module to monitor temperature and manage dosing. P20AF sets when the sensor circuit voltage is higher than expected, which can occur with an open signal line or a short to a positive supply.
Common causes
Most likely first.
- Open signal line
- Connector pins corroded - high contact resistance
- Supply wire pinched against B+ source
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
- Record freeze-frame data and inspect live metering-unit temperature and sensor signal for a high or fixed reading.
- Inspect the sensor connector for loose or corroded terminals and the harness for breaks, pinching, or contact with a positive supply wire.
- Check signal-wire continuity between the sensor and control module, since an open line can produce a high circuit reading.
- With power isolated as directed by the manufacturer, test for a short between the sensor signal and battery-positive wiring.
- Test the sensor and circuit against the manufacturer's specification to distinguish a failed sensor from an open or shorted wire.
Common fixes
- Reconnect or repair a loose or corroded sensor terminal.
- Repair an open signal wire in the metering-unit temperature sensor circuit.
- Repair wiring shorted to a positive supply.
- Replace the temperature sensor or repair the module only if testing confirms that component is faulty.
Can I keep driving?
The vehicle may still drive, but an incorrect temperature signal can impair DEF dosing and emissions control, and a restart countdown may appear. Follow any countdown and arrange diagnosis soon.
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.