What it means
DEF tank temperature sensor A reads within electrical range but plausibility fails — sensor sticks during a heating cycle.
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 DEF/AdBlue temperature so the control module can manage tank heating and determine whether reductant is available. P205B sets when the signal remains electrically plausible but its response does not match expected temperature behavior, such as failing to change appropriately during a heating cycle.
Common causes
Most likely first.
- Sensor drifts
- 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 compare the tank temperature reading with ambient temperature and available heating-cycle data.
- Watch live tank temperature data during a cold start or heating cycle to see whether it changes plausibly rather than sticking.
- Inspect the sensor connector and harness for loose terminals, corrosion, moisture, or damage that could damp or distort the reading.
- Check that the tank temperature sensor is securely mounted and positioned as intended.
- Test the sensor response and circuit against the manufacturer's specification before replacing the sensor.
Common fixes
- Secure the sensor if its mounting is loose.
- Clean or repair loose or corroded connector terminals.
- Repair wiring that causes an unstable or damped signal.
- Replace the reductant tank temperature sensor if its reading drifts or sticks outside the specified response.
Can I keep driving?
Driving is generally possible if there are no other symptoms, but inaccurate tank temperature data can interfere with DEF/AdBlue heating and emissions control, particularly in cold weather. Follow any emissions warning or start countdown because ignoring it can lead to 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.