What it means
The reductant (AdBlue/DEF) tank or line heater A does not reach the expected temperature within the calibrated time window. In cold weather frozen DEF blocks the SCR dosing system and the engine enters a no-restart state after ignored warnings.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater A warms the DEF (AdBlue) tank or a heated section of the reductant system so the fluid can be delivered in cold conditions. The control module checks the heater’s warming response, using temperature feedback, and sets this code when heater A does not bring its monitored area to the expected temperature within the allowed time.
Common causes
Most likely first.
- Heater element failed
- Tank temperature sensor drift
- Heater relay failed
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
- Use a scan tool to record freeze-frame data and compare the reductant temperature readings with ambient temperature before clearing the code.
- Check the DEF level and look for frozen fluid, leaks, or visible damage around heater A and its tank or line connections.
- Inspect heater A’s connector and wiring for corrosion, loose pins, chafing, or heat damage.
- With the scan tool, command heater A on where supported and watch its temperature response against the other available reductant temperature readings.
- Test heater A’s element and temperature sensor according to the manufacturer’s procedure to distinguish a weak heater from inaccurate temperature feedback.
- If the heater and sensor test correctly, check the heater relay and its control circuit under load against the manufacturer’s specification.
Common fixes
- Repair loose, corroded, or damaged heater A wiring and connector terminals.
- Replace a failed heater A element or heater assembly.
- Replace a temperature sensor that gives an implausible reading or fails its specified test.
- Replace a failed heater relay after confirming its supply and control circuits.
Can I keep driving?
If temperatures are above freezing and there are no drivability symptoms, a short trip may be possible, but arrange repair soon because the SCR system may not heat or dose DEF correctly. In freezing weather, heed any no-restart countdown; continued operation can leave the vehicle unable to restart and increase NOx emissions.
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.