P0172: System Too Rich (Bank 1)
Severity: medium
What it means: The engine computer is pulling fuel out because bank 1 is running with too much fuel for the amount of air.
Safe to drive? Usually drivable short-term, but a rich mixture can foul plugs and damage the catalytic converter.
Common symptoms
- Check-engine light
- Poor fuel economy
- Fuel smell or black smoke
- Rough idle
Likely causes, ranked
- Leaking fuel injector or high fuel pressureShop
- Faulty mass air flow sensor reading low$323DIY
CarMD 2024 national average about $323
- Faulty upstream O2 or air/fuel sensor$254Shop
CarMD 2024 national average about $254
- Fuel diluting the engine oil (some direct-injection turbo engines)Shop
Vehicle-specific notes
- 2016–2018 Honda Civic: 1.5L turbo fuel/oil dilution. Honda's warranty extension covers these misfire and rich codes with a software update. Read the full answer · Source
- 2017–2019 Honda CR-V: 1.5L turbo fuel/oil dilution. Honda's warranty extension covers these misfire and rich codes with a software update. Read the full answer · Source
What it means: The engine computer is pulling fuel out because bank 1 is running with too much fuel for the amount of air.
Severity: Medium. Safe to drive? Usually drivable short-term, but a rich mixture can foul plugs and damage the catalytic converter.
DIY or shop?
Shop diagnosis is recommended. Reading the fuel trims needs a scan tool.
Before you replace parts
- Read all stored codes and freeze-frame data. Fix misfire, fuel or sensor codes before chasing emissions codes.
- Check the simple things first: connectors, vacuum hoses, fluid levels and the gas cap.
- Clear the code only after a repair, then confirm it stays off after a few drives.
- Check your VIN for recalls and service bulletins.
Safety / mechanic note
Verify with a mechanic if you notice drivability changes, warning lights or overheating. A code points to a circuit or system, not always a specific part.