P22FF
SCR NOx Catalyst Inlet Temperature Too Low
The temperature sensor at the SCR catalyst inlet reports a value below the threshold needed for efficient ammonia conversion. Below roughly 180-200 °C the SCR cannot reduce NOx and reductant injection is suspended; if this is detected continuously the ECU sets the fault.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Common causes
Ordered by how often each turns out to be the cause.
- Temperature sensor drifts low or has shifted resistanceCommon
- Short-trip operation - exhaust never reaches operating temperaturePossible
- Exhaust heater or heat-management strategy not activeLess likely
Parts involved
- SCR Inlet Temperature Sensor
- SCR Catalyst
- Exhaust Heat Management
P22FF is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.