P0172 (system too rich, bank 1) and P0175 (system too rich, bank 2) are the opposite of a lean code: the engine computer had to remove a lot of fuel to keep the mixture correct, and hit its limit. Either the engine is getting too much fuel, or it is measuring too much air. The long-term fuel-trim number that triggers these codes is usually more negative than about −25%. Here is what causes it, how to isolate it, and typical repair costs.
Symptoms of a rich condition
Running rich often smells like fuel, fouls spark plugs (black, sooty), causes a rough idle or misfire, drops fuel economy, and can eventually damage the catalytic converter. Black smoke from the tailpipe on acceleration is a strong rich sign.
The most common causes, ranked
Leaking fuel injector(s) — the most common mechanical cause; an injector that drips adds fuel the PCM never commanded.
High fuel pressure — a failed fuel-pressure regulator or a pinched return line forces more fuel through the injectors than intended.
EVAP purge valve stuck open — a purge solenoid that sticks open continuously siphons raw fuel vapor from the charcoal canister into the intake. It's a common, cheap-to-fix cause; clamp the purge hose during diagnosis to see if the trims return to normal.
MAF sensor over-reporting — a contaminated or faulty MAF sensor that reads high air makes the PCM add matching fuel. Trims rich across idle and cruise point here.
Biased oxygen / air-fuel sensor, cold coolant signal, or restricted air — an O2 sensor that reads falsely rich makes the PCM pull fuel and can log a rich fault (confirm the sensor before replacing it); a coolant temp sensor stuck reading cold keeps the engine in warm-up enrichment; a clogged air filter starves airflow.
On direct-injection (GDI) engines, watch for fuel dilution of the engine oil: a worn high-pressure fuel pump can leak fuel past its plunger into the crankcase, and the PCV system then draws fuel vapor into the intake, producing stubborn rich trims on both banks. If the oil level is rising or smells strongly of gasoline, check that before pulling injectors.
How to diagnose it in order
Check whether one bank or both are rich. Both banks = a shared cause (fuel pressure, MAF, coolant temp signal); one bank = a bank-specific leaking injector.
Pull the spark plugs — uniformly black/sooty confirms a broad rich condition; one wet/black plug points to a single leaking injector.
Check fuel pressure with a gauge; compare to spec, and watch it after key-off for a slow bleed-down (leaking injector or regulator).
Verify the coolant temp sensor reads correctly at operating temperature.
Check MAF grams/sec against spec and the O2/AFR sensor's switching.
Repair cost (2026 US ranges)
Ranges assume a US independent shop at about $110–$160/hour plus typical parts. Planning figures, not a quote.
Repair
Shop (parts + labor)
DIY parts
Fuel injector (one)
$300–$700
$60–$300
Fuel pressure regulator
$200–$500
$40–$200
MAF sensor
$200–$450
$80–$300
Oxygen / air-fuel sensor
$200–$450
$60–$250
Coolant temp sensor
$120–$300
$20–$120
Air filter
$30–$80
$15–$40
Diagnosis
$100–$200
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FAQ
Is it safe to drive with a rich code?
A rich mixture washes oil off the cylinder walls, fouls plugs, and overheats the catalytic converter, so fix it promptly. Stop driving if the check engine light flashes.
What is the most common cause of P0172?
A leaking fuel injector or high fuel pressure from a bad regulator are the usual mechanical causes; a contaminated MAF sensor is the most common electrical cause. Check plug color and fuel pressure first.
How much does it cost to fix a rich code?
From about $30 for an air filter to $300–$700 for a leaking injector. A cleaning of the MAF sensor may cost nothing. Diagnose the cause first so you replace the right part.
General diagnostic information for US-market vehicles, not a substitute for a hands-on inspection. Costs are 2026 ballpark ranges for planning only. Always follow the correct procedure for your specific vehicle.