P0172: System Too Rich (Bank 1)

P0172 means System Too Rich (Bank 1). This code means bank 1 (the side with cylinder 1) is running rich (too much fuel or too little air) and the engine computer has reached the limit of how much fuel it can remove.

CodeP0172
SystemFuel trim
SeverityModerate
TypeGeneric powertrain

Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.

Common symptoms

  • Check engine light on
  • Poor fuel economy
  • Fuel smell from the exhaust
  • Black smoke or sooty spark plugs
  • Rough idle

Possible causes

  • Leaking fuel injector
  • High fuel pressure or faulty fuel pressure regulator
  • MAF sensor over-reporting air flow
  • Coolant temperature sensor reading too cold
  • EVAP purge valve stuck open
  • Engine oil diluted with fuel

How to diagnose P0172

  1. Read fuel trims and compare both banks.
  2. Check fuel pressure and whether it holds after shut-off (leak-down).
  3. Check MAF readings against expected values at idle.
  4. Compare coolant temperature reading with actual engine temperature.
  5. Pinch off or command the purge valve closed and watch if trims change.
  6. Smell the oil dipstick for fuel.

What the code means, digit by digit

PPowertrain0Generic (SAE) code1Sub-system72Specific fault

Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.

Symptoms linked to P0172

Black smoke from the exhaustSmell of fuelPoor fuel economy

Related codes

General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.