P0172 System too rich (bank 1)
What P0172 means
The engine computer has had to reduce fuel on bank 1 as far as it is allowed and the mixture is still too rich. Unburned fuel passes into the exhaust, which wastes fuel and can overheat and damage the catalytic converter.
P0172 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- High
- Typical repair cost
- 50–600 €
- Workshop time
- 0.5–4 h
- Repair difficulty
- Medium
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Higher fuel consumption
- Black smoke on acceleration
- Smell of unburned fuel
- Fouled spark plugs, possible misfires
Common causes
- Leaking or dripping fuel injectors
- Fuel pressure too high from a faulty regulator
- Faulty mass air flow sensor over-reading airflow
- Coolant temperature sensor reading too cold
- EVAP purge valve stuck open
Can I keep driving with P0172?
Get it to a workshop within a few days. Running rich for a long time can damage the catalytic converter and foul the spark plugs.
What to check first
- Read the freeze-frame and live fuel trims to confirm strong negative correction and when it occurs (idle, load, cold).
- Check the coolant temperature reading matches the real engine temperature.
- Check for a stuck-open purge valve and test fuel pressure against the specification.
- Test the MAF sensor and check the injectors for leaks.
Related codes
- P0171 System too lean (bank 1)
- P0173 Fuel trim malfunction (bank 2)
- P0170 Fuel trim malfunction (bank 1)
- P0174 System too lean (bank 2)
- P0175 System too rich (bank 2)
- P0181 Fuel temperature sensor A range/performance
- P0161 Oxygen sensor heater circuit (bank 2, sensor 2)
- P0160 Oxygen sensor circuit no activity detected (bank 2, sensor 2)
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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