P0172 Engine Running Rich (Bank 1)

What it means

The engine computer adjusts fuel all the time to keep the air-fuel mixture right. P0172 means that on bank 1 it has taken away as much fuel as it is allowed to, and the mixture still reads rich. Something is adding fuel or hiding air from the computer.

The technical version

Fuel trim is the correction the computer applies on top of its base fuel map. Short-term trim reacts in seconds; long-term trim learns over time. P0172 sets when the total trim on bank 1 stays at its negative limit. A MAF that reads too much air, a leaking injector, high fuel pressure, fuel vapor from a stuck-open purge valve, or a front oxygen sensor reading lean can all push it there.

Official name: Fuel Trim too Rich (Bank 1).

Symptoms

• Worse fuel economy.
• A fuel smell, especially at idle.
• Black soot at the tailpipe.
• Rough idle or misfires on some cars.

Causes, most likely first

How likely is our ranking from repair data and common practice, not a published rate.

What the live data shows

Watch long-term and short-term fuel trim for bank 1 at idle and at about 2,500 rpm. Strongly negative numbers confirm the code. If the other bank is also negative, look at things both banks share: the MAF, fuel pressure, the purge valve. If only bank 1 is rich, look at its injectors and its oxygen sensor. Coolant temperature should read near outside temperature after the car sits overnight.

What to check first, in order

1. Read fuel trims for both banks at idle and at 2,500 rpm.

   It tells a shared cause from a one-bank cause.

   What the data shows: Long-term trim near its negative limit confirms the code.

   Cost: no cost if you have a scanner

2. Check the air filter and clean the MAF sensor with MAF cleaner.

   A dirty MAF is a common, cheap cause.

   What the data shows: Trims should move back toward 0% if the MAF was the cause.

   Cost: a can of MAF cleaner

3. Pinch or unplug the purge hose with the engine idling (or have a shop test the purge valve).

   A stuck-open purge valve feeds fuel vapor.

   What the data shows: Trims recover when the purge path is closed.

   Cost: free to look

4. Check the fuel pressure regulator and fuel pressure.

   High pressure adds fuel on both banks.

   What the data shows: Not a standard OBD reading on most cars.

   Cost: $331-$422 at a shop (Fuel pressure regulator replacement, RepairPal)

5. Test or replace a leaking injector.

   A dripping injector makes one bank rich.

   What the data shows: Misfire counts and trims return to normal.

   Cost: $768-$953 at a shop (Fuel injector replacement, RepairPal)

Is it safe to drive?

Get it checked soon

Yes, fix soon. Extra fuel wastes money and can overheat the catalytic converter.

Usually drivable for a short time. Expect worse fuel economy, maybe a fuel smell and black soot at the tailpipe. Left alone, a rich mixture fouls spark plugs and can overheat the catalytic converter, which is the expensive part to replace. A raw fuel smell or a leak is a reason to stop.

Stop driving and get help if

• The check engine light flashes.
• You smell raw fuel strongly or see fuel leaking.
• The engine misfires or stalls.

What it costs

National averages for the full job at a shop, parts and labor, from RepairPal (updated June 18 and September 3, 2026). Your price depends on the car and the area. Many of the first checks on this page cost nothing with a scanner. MAF sensor replacement was CarMD's number 4 check engine repair in 2025, at an average of $346. Oxygen sensor replacement was CarMD's number 2 check engine repair in 2025, at an average of $287. CarMD averages come from a different method (repairs reported by its users), so they can sit below RepairPal's shop estimates.

Sources for the notes: CarMD 2026 Vehicle Health Index (press release, April 22, 2026).

Can I clear it? Will it come back?

After the repair, clear the code and drive. Long-term fuel trim relearns over a few drives; the code should not return.

Will it fail an emissions test?

Yes, while the check engine light is on, most OBD emissions tests fail the car.

Carmaker notes

Service bulletins and warranty extensions that name this code. Ask a dealer to check your VIN.

• Toyota 2009-2018 Sequoia and 2009-2019 Tundra with the 3UR-FBE flex-fuel engine

  Toyota bulletin T-SB-0166-19 (December 2019) covers P0172 and P0175, as well as P0171 and P0174, when the engine estimates the alcohol content of the fuel wrongly.

  Toyota T-SB-0166-19 via NHTSA static.nhtsa.gov

Related codes

Questions people ask

Is P0172 serious?

Not urgent, but do not wait long. A rich mixture can damage the catalytic converter over time.

Can a bad O2 sensor cause P0172?

Yes, but less often than a MAF, a leaking injector or the purge valve. Check fuel trims on both banks first.

Will P0172 make me fail an emissions test?

Yes, while the check engine light is on.

Sources

1. RepairPal: P0172 code page repairpal.com
2. RepairPal: Mass airflow (MAF) sensor replacement (updated September 3, 2026) repairpal.com
3. RepairPal: EVAP purge valve replacement (updated September 3, 2026) repairpal.com
4. RepairPal: Fuel pressure regulator replacement (updated June 18, 2026) repairpal.com
5. RepairPal: Fuel injector replacement (updated September 3, 2026) repairpal.com
6. RepairPal: Oxygen sensor replacement (updated September 3, 2026) repairpal.com
7. CarMD 2026 Vehicle Health Index (press release, April 22, 2026) prnewswire.com
8. Toyota T-SB-0166-19 via NHTSA static.nhtsa.gov
9. Texas DPS emissions testing FAQ dps.texas.gov
10. Arizona myazcar.com: OBD and the check engine light myazcar.com