Meaning
The computer reduces the injection to clean up the mixture and has reached the correction limit. More fuel than air is entering the cylinders.
Urgency
Average. The car is driven, but the fuel consumption increases, and the unburnt fuel damages the catalyst and the lambda probes.
It affects
Gasoline engines. Nozzles, fuel pressure, flowmeter, lambda probe, air filter.
What does P0172 mean
The lambda sensor in front of the catalytic converter reads too little oxygen in the exhaust gas. The computer shortens the injection, but when the correction exceeds about −25 %, it records P0172. The reverse — a lean mixture — is P0171.
Symptoms
- Increased fuel consumption
- The smell of fuel, black smoke from the exhaust
- Uneven idle, choppy
- Hard to start a warm engine
- Soot on the candles
| Can it be driven?
Yes, briefly. Do not postpone the repair - the rich mixture overheats and clogs the catalyst (P0420), which is the more expensive repair.
Common reasons
- Leaking or stuck open nozzle.
- Fuel pressure too high — defective regulator.
- Defective flow meter (MAF) — reports more air than actual (P0101).
- Clogged air filter.
- Slow or faulty lambda probe (P0133).
- Fuel in the oil or in the fuel vapor system — suction through crankcase ventilation or EVAP valve.
- Faulty engine temperature sensor — the computer "thinks" the engine is cold.
How we diagnose it
- We look at the fuel corrections and lambda probe signal in real time.
- We check the air filter and flow meter.
- We measure the fuel pressure and check the nozzles for leaks.
- We check the temperature sensor and the fuel vapor system.