What P0172 means
The engine on bank 1 is getting more fuel than it can burn, and the computer has already cut fuel as far as it's allowed. A common cause is a purge valve stuck open, which lets fuel vapor from the tank flow into the engine all the time. Leaking injectors or too-high fuel pressure can do the same.
How the engine computer detects it
With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, negative values mean fuel is being removed; around ±10 % is normal. P0172 means long-term trim on bank 1 is at its lower limit: the module already removes the maximum fuel and the mixture still stays rich. GM separates purge influence carefully: before the first purge it evaluates passively (factor 0.70 or below, i.e. −30 %), afterwards it closes the purge valve briefly for an active test, so a full charcoal canister is not counted as a fault. Ford only evaluates with purge off, at −25 %. Two trips. Bulletins name an open purge valve, leaking direct injectors and, on flex-fuel vehicles, a wrongly learned ethanol content. A permanently rich mixture can overheat the converter and dilutes the oil.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on (after two drive cycles)
- Higher consumption, fuel smell
- Black smoke, sooted spark plugs
- Rough idle or starting problems after refuelling; converter damage possible later
Common causes
- Purge valve stuck open or charcoal canister saturated Most likely
- Leaking or dripping injectors (also direct injection) Possible
- Fuel pressure too high (faulty regulator, blocked return, regulator vacuum hose) Possible
- Fuel in the engine oil (short trips, leaking injectors), vapour via crankcase ventilation Possible
- Clogged air filter or intake, air flow sensor reading high; flex-fuel: wrongly learned ethanol content Less likely
How to diagnose P0172
- Check for P0175 (bank 2 rich) and misfire codes. If both banks are rich, look for causes they share, like the purge valve or fuel pressure.
- Check the oil level and smell the dipstick. If the level is rising or the oil smells of gasoline, fuel is getting into the oil and an oil change is due once the cause is fixed.
- Look at the air filter and intake for a blockage. It's quick to rule out, though rarely the whole story.
- Test the EVAP purge valve. On most cars it should not let air through with the engine off and the valve unplugged; if it does, it's stuck open.
- Have fuel pressure measured, and ask the shop to check whether pressure holds after shutdown. Pressure that drops fast can mean a leaking injector.
Vehicles that report P0172 most
| Vehicle | Years | Reports | Share | Median miles |
|---|---|---|---|---|
| Honda CR-V | 2016–2020 | 53 | 57k | |
| Honda Accord | 2018–2024 | 5 | — | |
| Chevrolet Equinox | 2010–2015 | 4 | — | |
| Kia Sorento | 2013–2017 | 3 | — | |
| Honda Civic | 2018–2021 | 3 | — | |
| Honda Pilot | 2016–2019 | 3 | — |
Counts of owner complaints to the NHTSA that mention P0172 (96 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 1
- Fuel and air metering
- 72
- Specific fault
Questions drivers ask
Can I drive with code P0172?
Yes, for a short time. You'll burn more fuel, and running rich for weeks can foul spark plugs, thin the engine oil with gasoline and overheat the catalytic converter. If the light starts flashing or the engine misfires, stop driving.
Will my car pass an emissions inspection with P0172?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0172 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
Can I fix P0172 myself?
Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.
Often reported together
Codes that owners mention in the same NHTSA complaints as P0172. They can point to a shared cause, so it's worth checking them together.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.