NOx Exceedance - SCR NOx Catalyst Performance
NOx emissions are above the legal threshold and the ECU isolates the cause to the SCR NOx catalyst. Conversion efficiency is degraded — aged substrate, sulphur poisoning, or thermal damage.
Affected components
- Scr catalyst
- Reductant dosing unit
Common causes
- High likelihood: SCR catalyst substrate aged or contaminated
- Medium likelihood: Thermal damage from prior over-temperature event
Possible symptoms
- Check engine light on
- Failed emissions inspection
- Possibly slightly increased fuel consumption
- Sulfur (rotten egg) smell from exhaust under load
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
- Reduced-power or limp mode may occur
Sources
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.