Engine codes
P0172: system too rich (bank 1)
P0172 is the opposite of a lean code: the computer has pulled as much fuel as it may from bank 1, and the oxygen sensor still reports a mixture with too much fuel and too little oxygen.
P0172: can I drive, and what will the labor cost?
Estimated labor, replace a leaking fuel injector
$110 to $330
| Labor time (estimate) | 1 to 3 h |
| Rate used | $110/h |
| Technician wage, United States (average) | $27.13/h |
| Moderate: fix within weeks Drivable short term, but a rich engine fouls spark plugs and overheats the catalytic converter; fix within a couple of weeks. | |
Labor only: parts and the diagnostic fee are not included. Hours are our estimate for a typical car; book times vary by model. Wage: BLS, May 2025.
P0172 is the generic OBD-II code for "system too rich, bank 1". The fuel system monitor sets it when long-term fuel trim reaches its lower limit, the most fuel the computer is allowed to remove, and the upstream oxygen sensor still reads rich. Either extra fuel is getting in that the computer did not command, or the computer is being told the engine needs more fuel than it does. A leaking or stuck-open injector, fuel pressure that is too high because of a failed regulator or a pinched return line, an air flow sensor that over-reads and a coolant temperature sensor that reads cold, making the computer think the engine needs cold-start enrichment, are the main causes. A purge valve stuck open, which feeds fuel vapor into the intake, can add to it. A rich engine wastes fuel, smells of it, fouls spark plugs and overheats the catalytic converter, so it should be fixed within a couple of weeks.
Read your fuel trims (from a scanner's live data)
Total trim at idle / 2,500 rpm
+20% / +6%
| Reading: Lean at idle that improves with engine speed: the pattern of a vacuum leak. |
Positive trims mean the computer is adding fuel. This idle-versus-speed comparison is a common shop test, not a rule: confirm with a smoke test or a fuel pressure gauge. Negative values can be typed with a minus sign.
Codes, rules and labor data checked by Radif Partners · Editorial policy · Method and sources
P0172 at a glance
| Code | P0172 |
| Standard description | System Too Rich (Bank 1) |
| System | Fuel mixture and delivery |
| Severity | Moderate: fix within weeks |
| Common symptoms | fuel smell, black smoke, poor mileage, rough idle |
Signs that point to a cause
- Black smoke, a raw fuel smell and hard hot starts suggest a leaking injector or high fuel pressure. Some engines drip fuel into a cylinder after shut-off, which also causes a misfire on start.
- Poor running only when cold, with a temperature gauge that rises normally, points to the coolant temperature sensor; a scanner reading far below the real temperature confirms it.
- A rough idle right after refueling points to the purge valve, which should stay closed at idle.
- A dark sooty oil smell can mean fuel diluting the oil, which a shop checks when injectors are suspected.
Why the catalytic converter cares
Unburned fuel reaches the converter and burns there, raising its temperature far above normal. Over weeks that can melt or crack the substrate and lead to P0420. A rich condition is therefore cheaper to fix early. The fuel trim reader on this page shows how strongly your engine is correcting; negative values mean fuel is being removed.
On a V engine, a shared cause such as fuel pressure sets P0172 and P0175 together, while a single leaking injector usually sets only the code of its bank and may set a misfire code for that cylinder. Compare with P0171, the lean version.
Likely causes of P0172, in order
| Cause | How often |
|---|---|
| Leaking or stuck-open fuel injector | most frequent |
| Fuel pressure too high: regulator or return line | frequent |
| MAF sensor over-reading | frequent |
| Coolant temperature sensor reading cold | frequent |
| EVAP purge valve stuck open | frequent |
| Badly restricted air filter | less frequent |
P0172 repairs and labor time
| Repair | Labor (estimate) | At $110/h |
|---|---|---|
| Replace a leaking fuel injector | 1 to 3 h | $110 to $330 |
| Replace the coolant temperature sensor | 0.3 to 1 h | $33 to $110 |
| Replace EVAP purge valve | 0.3 to 1 h | $33 to $110 |