When an engine is cold, the ECU intentionally enriches the mixture to aid starting and warm-up. This is called cold start enrichment or SAI (Secondary Air Injection) operation on some systems. The gas readings during this phase are not representative of normal operating conditions and should not be diagnosed as a fault.
How to Identify Cold Enrichment
Check these signs:
- Engine coolant temperature (via OBD) is below 60°C (140°F)
- High CO (often > 2%) and low O₂ (near 0%)
- SAI active — some systems show SAI status in live data
- As engine warms, CO should drop and O₂ should rise toward stoichiometric
If these rich readings persist after the engine reaches normal operating temperature (90°C coolant), then it's a genuine rich fault requiring diagnosis.
What Can Go Wrong?
Actual rich faults while warm include:
- Leaking fuel injector
- Faulty fuel pressure regulator
- Bad coolant temperature sensor (telling ECU engine is still cold)
- Stuck-open purge valve (introducing fuel vapors)
- MAF overestimation (ECU adds fuel based on false high airflow)
To distinguish these, check fuel trims: both STFT and LTFT negative indicate a compensating rich condition.
Practical Check
- Warm up the engine fully (radiator fan should cycle)
- Retake 5-gas measurements
- Compare to cold-start values
- If readings normalize (CO < 1%, lambda ~1.0), cold enrichment was the cause
The 4D diagnostic engine automatically relaxes O₂ thresholds when the "Cold Engine" checkbox is enabled, recognizing that cold enrichment affects gas signatures.