NOx forms at high combustion temperatures. Rich mixtures typically reduce NOx because the excess fuel absorbs heat and lowers peak temperatures. So if you see high CO (rich) and high NOx, something unusual is happening.
Possible Explanations
1. Lean Bank in a misfiring cylinder
If one cylinder is running extremely lean (or misfiring), the other cylinders may be compensating by running even richer to keep overall lambda near 1.0. The lean cylinder can produce high local temperatures, generating NOx, while the overall gas reading shows rich due to the other cylinders. This often accompanies high HC (misfire).
2. Faulty ECT Sensor
An ECT reading colder than actual causes the ECU to add fuel (enrichment) but also advances timing to compensate for the perceived "cold engine." Advanced timing raises combustion temperatures, potentially producing NOx despite the rich mixture.
3. High Load with Rich WOT
Under wide-open throttle, some systems enrich slightly for power while cylinder pressures and temperatures are very high. This can produce measurable NOx even with CO present.
4. Advanced Timing With Rich Fault
If there's both a rich condition (fuel delivery issue) and ignition timing that's too advanced, the resulting high temperatures can generate NOx despite the rich overall mixture.
Diagnostic Steps
- Check for misfires (high HC, cylinder contribution data if available)
- Verify ECT sensor accuracy (compare to known-good scan tool or infrared thermometer)
- Look at timing advance PID if accessible
- Consider running at different loads to see if pattern changes
Conclusion
High NOx with rich readings is a red flag that something is counteracting the NOx-suppression effect of richness. Look for lean cylinders, timing issues, or sensor errors before concluding the mixture is simply "rich."