What it means
The metering-unit temperature sensor signal is outside the expected range or does not respond as expected. The code does not by itself establish a wiring fault.
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 reports its temperature to the control module for dosing and protection decisions. P20AD sets when the reading is outside the expected range or fails to change as expected; this points to an implausible response but does not by itself identify a wiring fault.
Common causes
Most likely first.
- Sensor drift
- Sensor element aged beyond calibration window
- Sensor mounting loose - signal damping
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 from a cold start through warm-up to see whether the reading changes plausibly.
- Inspect that the sensor is securely mounted and its connector is fully seated, since loose mounting or contact can distort the response.
- Compare the reported temperature with an independent measurement under the same conditions, following the manufacturer's procedure.
- Check the sensor signal and wiring for intermittent changes while gently inspecting the harness and connector.
- Test the sensor response against the manufacturer's temperature-versus-signal specification before replacing parts.
Common fixes
- Secure the sensor mounting and connector if either is loose.
- Repair connector or wiring damage found during inspection.
- Replace the temperature sensor if its reading drifts or response fails the manufacturer's specification.
- Replace the metering unit only if the sensor cannot be serviced separately and testing confirms its failure.
Can I keep driving?
Driving is generally possible if there are no other symptoms, but an implausible temperature reading can impair DEF dosing and emissions control. Watch for a restart countdown and have the fault diagnosed 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.