P0172 means System Too Rich (Bank 1). The computer has removed as much fuel as it can on bank 1 and the mixture still reads rich, so the engine is getting too much fuel or too little air. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0172 means
The engine computer keeps adjusting fuel to hold the air-fuel mixture near the ideal ratio, using oxygen sensor feedback. Those adjustments are fuel trims: short-term trim reacts instantly and long-term trim learns over time. Positive numbers mean the computer is adding fuel; negative numbers mean it is taking fuel away. When long-term trim sits near the limit the computer allows, it sets a lean or rich code. On a V engine, bank 1 is the side containing cylinder 1 and bank 2 is the opposite side; inline engines only have bank 1.
P0172 symptoms
- Black smoke and a fuel smell
- Poor fuel economy
- Rough idle or misfires from fouled spark plugs
Common causes of P0172
- Leaking fuel injector
- Fuel pressure too high from a faulty regulator
- MAF sensor reporting more airflow than is actually entering the engine
- Clogged air filter or intake restriction
- Faulty coolant temperature sensor making the engine think it is cold
How to diagnose P0172
- Check fuel trims and fuel pressure
- Inspect the air filter and intake
- Compare coolant temperature on the scan tool with the actual engine temperature
- Check for leaking injectors
Can you drive with P0172?
Short trips are usually possible, but a rich mixture fouls spark plugs, dilutes oil with fuel and can overheat the catalytic converter.
P0172 tips
Black, sooty spark plugs on that bank confirm a rich condition. A fuel smell on the oil dipstick means fuel is diluting the oil, so change the oil once the cause is fixed. Rich codes can also come from a coolant temperature sensor that reads cold, so compare its reading with the real engine temperature.
Will P0172 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0172 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0175 | System Too Rich (Bank 2) |
| P0171 | System Too Lean (Bank 1) |
| P0101 | Mass or Volume Air Flow Circuit Range/Performance |
| P0117 | Engine Coolant Temperature Sensor 1 Circuit Low |
Guides that help
- fuel trim explained
- fuel pressure regulator
- fuel injector symptoms
- fuel pump, filter and regulator symptoms
- MAF sensor symptoms and cleaning
- exhaust smoke colors
When it is more than a sensor
Fuel trim codes are almost always fixed with a leak repair, a sensor or a fuel delivery part. If a long-running lean condition has burned a valve or piston and a compression test confirms it, compare repair against a replacement on our engines page and see engine replacement cost.
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