What it means
A general electrical fault is reported on the temperature sensor inside the DEF metering unit — used to protect the unit from over-temperature and to compensate for fluid viscosity.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
A temperature sensor in the DEF metering unit reports the unit's temperature to the control module, which uses the reading to monitor operating conditions and adjust or protect dosing. P20AC indicates a general electrical fault in this sensor circuit, without specifying whether the signal is too high, too low, or outside its expected range.
Common causes
Most likely first.
- Sensor element failed
- Harness fault
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 check live metering-unit temperature to see whether the reading is missing, erratic, or implausible for the vehicle's condition.
- Inspect the sensor connector and nearby harness for loose terminals, corrosion, damaged insulation, or a poor connection.
- Check sensor wiring continuity and for shorts as directed by the manufacturer's diagnostic procedure.
- Compare the sensor reading with an independent temperature check under the same conditions, following the manufacturer's procedure.
- Test the sensor and control-module circuit against the manufacturer's specification to identify whether the fault is in the sensor, wiring, or module.
Common fixes
- Reconnect the sensor plug securely and repair any damaged connector terminals.
- Repair an open, shorted, or damaged section of sensor wiring.
- Replace the metering-unit temperature sensor if its output fails the manufacturer's test.
- Repair or replace the control module only if the circuit and sensor test correctly and module fault is confirmed.
Can I keep driving?
You can usually drive if the vehicle behaves normally, but inaccurate metering-unit temperature data can affect DEF dosing and emissions, and a restart countdown may appear. Follow any dashboard 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.