Every petrol engine runs rich at cold start — the ECU compensates for fuel condensing on cold cylinder walls. Misreading this as a rich-mixture fault is an easy mistake on an analyzer that isn't showing ECT.
Normal cold-start pattern
- CO elevated (1–4%) for the first 1–2 minutes.
- HC elevated and dropping toward < 200 ppm within a minute.
- CO₂ climbing as combustion stabilises.
- O₂ variable depending on SAI state.
Actual rich fault
- Rich condition persists past 5 minutes of running.
- STFT consistently negative (ECU removing fuel).
- CO remains > 1% after full warm-up.
- λ < 0.9 at hot idle.
Diagnostic flow
- Let the engine reach full operating temperature (coolant > 80°C, oil warm).
- Wait for SAI cycle to end.
- Record stabilised idle gases.
- If CO still > 0.5% and λ < 0.95 — investigate enrichment causes (injector leak, stuck coolant temp sensor, fuel pressure high).
// common red herring
A stuck-low coolant temperature sensor tells the ECU the engine is cold forever. Enrichment never exits and you see permanent rich idle. Check ECT live-data before chasing fuel system parts.