What it means
A general electrical fault is reported on DEF tank temperature sensor A — separate from the metering-unit temperature sensor and used to gate tank-heating.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
The reductant tank temperature sensor measures the temperature of DEF/AdBlue in the storage tank, helping the control module manage tank heating and reductant availability. P205A indicates a general electrical fault in the sensor circuit, so the reading or circuit response is not behaving as expected.
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 compare the tank temperature reading with ambient conditions and other available temperature data.
- Inspect the tank temperature sensor connector and harness for damage, water ingress, corrosion, or loose terminals.
- Check that the sensor is mounted securely and that its connector is fully seated.
- Use the wiring diagram to check sensor power or reference, ground, and signal circuit continuity as applicable to the vehicle.
- Test the tank temperature sensor against the manufacturer's specification and repair the confirmed circuit or sensor fault.
Common fixes
- Reconnect a loose sensor connector and secure the harness.
- Clean or repair corroded or water-damaged terminals.
- Repair damaged sensor wiring or a poor ground.
- Replace the reductant tank temperature sensor if it fails the specified test.
Can I keep driving?
You can generally drive if the vehicle behaves normally, but the system may misjudge DEF/AdBlue temperature and tank heating, especially in cold weather. Monitor emissions warnings and any start countdown, as the SCR system may eventually impose a no-start restriction.
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.