P0172: System Too Rich (Bank 1)
The engine computer has been pulling fuel out on bank 1 because the exhaust shows too much fuel and not enough air.
What P0172 means
P0172 is the opposite of a lean code. The oxygen sensor on bank 1 keeps reporting rich exhaust, so the computer keeps cutting fuel. RepairPal notes that the code sets when long term fuel trim falls to about minus 15 to minus 30 percent and the computer cannot cut any further. Bank 1 is the side with cylinder 1. Because the computer is already removing fuel, the cause is something adding fuel it did not ask for, or a sensor fooling it into adding more. A leaking injector or high fuel pressure adds fuel directly. A MAF sensor reading too high, or a coolant sensor saying the engine is cold, makes the computer command extra fuel. A stuck open purge valve feeds fuel vapor from the tank. Shops compare fuel trims at idle and cruise, watch MAF and coolant readings, and run a fuel pressure and leak down test. Fix it soon, since extra fuel washes oil off cylinder walls and can overheat the converter.
Common symptoms
- Check engine light on
- Poor gas mileage
- Strong fuel or rotten egg smell from the exhaust
- Black smoke or soot at the tailpipe
- Rough idle or misfires
Most common causes
- Leaking or stuck open fuel injector
- Fuel pressure too high from a bad pressure regulator
- MAF sensor reading too high
- Engine coolant temperature sensor reading falsely cold
- EVAP purge valve stuck open
- Badly clogged air filter
- Faulty front oxygen sensor
What to check yourself first
Check the engine air filter and replace it if it is packed with dirt. Look at live data for coolant temperature on a warm engine; it should read around 190 to 220 degrees F. On engines with a vacuum operated fuel pressure regulator, pull its vacuum hose and check for fuel inside, which means the regulator is leaking.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Fuel pressure test | $60-$88 | Checks for high fuel pressure or a leaking regulator. |
| Replace fuel pressure regulator | $317-$409 | If pressure is high or the regulator vacuum line smells of fuel. |
| Replace fuel injector | $768-$953 | When an injector leaks down or fails a balance test. |
| Replace EVAP purge valve | $210-$251 | A purge valve stuck open pulls fuel vapor into the engine at idle. |
| Replace engine coolant temperature sensor | $499-$585 | If the sensor shows a cold reading on a fully warm engine. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P0175 System Too Rich (Bank 2)
- P0171 System Too Lean (Bank 1)
- P0101 Mass or Volume Air Flow "A" Circuit Range/Performance
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0300 Random/Multiple Cylinder Misfire Detected