P0172 fault code: what it means

P0172: Bank 1 is running rich (too much fuel) and the engine computer has run out of correction.

Engine and gearbox (powertrain) · Costly if missed

Usual causes, most likely first

dirty or over-reading MAF sensor
leaking fuel injector
high fuel pressure
faulty front O2 sensor

P0172 by make

Audi

Engine running rich on bank 1. On direct-injection petrols, fuel getting into the oil from the high-pressure pump or leaking injectors can set it.

Usual causes: leaking or dirty fuel injectors; fuel in the oil from a leaking high-pressure pump; EVAP purge valve stuck open; excessive fuel pressure

BMW

Engine running rich, bank 1. On BMW direct injection engines leaking piezo injectors are a common cause and can wash the bore if left.

Usual causes: leaking fuel injector (N53 / N54 / N55 piezo); fuel pressure too high (regulator); oxygen sensor fault

Honda

Engine running rich (bank 1). On Honda 1.5 turbo (L15B) and newer hybrids, leaking direct injectors and fuel-diluted oil are well known causes.

Usual causes: leaking fuel injectors (1.5 turbo); fuel in the oil (oil dilution) on early 1.5 turbo; dirty MAF sensor or blocked air filter; fuel pressure fault

Hyundai

Engine running rich (too much fuel) on bank 1.

Usual causes: dirty or faulty MAF sensor; faulty front oxygen sensor; EVAP purge valve stuck open; leaking injector

Kia

Engine running rich (bank 1).

Usual causes: faulty oxygen sensor; leaking fuel injector (2.5 Smartstream); fuel diluted engine oil from short trips; vacuum leak skewing MAF reading

Mazda

Fuel trim too rich on SkyActiv-G. Usually a sensor reading wrong or the high-pressure fuel pump relief valve letting pressure run high.

Usual causes: MAF, ECT or MAP sensor reading wrong; high-pressure fuel pump relief/spill valve; leaking fuel injector; purge valve stuck open

Mercedes-Benz

Engine running rich, bank 1. On Mercedes usually the MAF sensor reading wrong.

Usual causes: dirty or faulty MAF sensor; leaking or clogged fuel injectors; vacuum leak

Mini

Engine running rich. On direct injection Cooper S engines (N14 / N18) a leaking injector is one likely cause.

Usual causes: leaking fuel injector (N14 / N18); fuel pressure too high; oxygen sensor fault

Skoda

Engine running rich on bank 1. On direct-injection petrols, fuel getting into the oil from the high-pressure pump or leaking injectors can set it.

Usual causes: leaking or dirty fuel injectors; fuel in the oil from a leaking high-pressure pump; EVAP purge valve stuck open; excessive fuel pressure

Volkswagen

Engine running rich on bank 1. On direct-injection petrols, fuel getting into the oil from the high-pressure pump or leaking injectors can set it.

Usual causes: leaking or dirty fuel injectors; fuel in the oil from a leaking high-pressure pump; EVAP purge valve stuck open; excessive fuel pressure

What the repair usually costs

Air-fuel ratio / front O2 sensor replace (upstream)NZ$270–NZ$750 · A$260–A$730
O2 sensor replace (downstream / after cat)NZ$180–NZ$480 · A$180–A$460
MAF (mass air flow) sensor replaceNZ$170–NZ$660 · A$160–A$600
Parking sensor replace, one sensorNZ$130–NZ$520 · A$130–A$510

Not sure what P0172 means on your car? Ask Credov.

Before you clear it

Write the code down or photograph the scanner first. A code that comes straight back after clearing points to a fault that is still there. A car whose codes were cleared just before you arrived may be hiding one: the scanner's readiness monitors show "not ready" for a while after codes are cleared.

Researched by Credov from manufacturer and workshop sources. A fault code points to where a fault may be; a workshop confirms it with a diagnosis. Browse all fault codes.