P204FPowertrain
Powertrain · SAE
Reductant System Performance
Meaning
The SCR/AdBlue system is not delivering the performance required for NOx reduction. The ECU detects dosing operating out of specification.
Common causes
- Dosing fault or faulty reductant injector
- Poor-quality reductant or the wrong fluid in the AdBlue tank
- Faulty NOx sensor
- Reductant pump or insufficient pressure
- Crystallised or blocked lines/injector
- Aged SCR catalyst
Symptoms
- Check-engine light, emissions-system message
- Reduced power
- Restart-limit countdown
- Failed emissions check
- Abnormal AdBlue consumption
Severity
Moderate to high: the emissions system is not working correctly and the vehicle may limit restarts.
What to do
- Check the AdBlue level and quality and rule out wrong fluid in the tank
- Read live data for dosing and the upstream and downstream NOx sensors
- Inspect the dosing injector, pump and lines for blockage/crystallisation
- Replace the faulty NOx sensor or injector and restore correct dosing