NOₓ forms at high combustion temperatures. Rich mixtures cool combustion, so in principle rich + high-NOₓ shouldn't coexist. When they do, the analyzer is telling you the measurement is masking a more complex picture.
Why rich normally lowers NOₓ
Excess fuel absorbs heat of vaporisation and reduces peak combustion temperature. With less heat there is less N₂ dissociation and NOₓ formation falls. Classic rich idle therefore shows high CO, low NOₓ.
High NOₓ + high CO — how
- Individual lean cylinder: mixture is rich overall (high CO from rich cylinders) while one or two lean cylinders are running hot and producing NOₓ.
- ECT sensor reporting cold: ECU enriches (high CO) while timing map stays cold-start (retarded) — hotspots still form in specific chambers.
- Oil burning: combustion of oil raises both CO and chamber temperature in localised spots.
- Misfire + load: raw fuel in the exhaust over-reads as CO on some analyzers; NOₓ from working cylinders remains normal.
Confirmation path
- OBD: cylinder-balance test or per-cylinder STFT.
- Cylinder drop: disable injectors one at a time and watch rpm response.
- Compression / leakdown if cylinder imbalance is confirmed.
- Spark plug inspection — colour asymmetry across cylinders is diagnostic.