P0172 System Too Rich Bank 1
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.
Likely causes
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
Repair estimate
Typical range across vehicles; the real cost depends on what the testing finds.
What to check first
- Check fuel trims
- Check fuel pressure regulation
- Inspect air filter
- Test injectors for leaks
Parts involved
Related codes
References: law.cornell.edu, gsi.ext.gm.com, fordservicecontent.com, saabwisonline.com.