P0172
System Too Rich (Bank 1)
medium
i Description
The engine is running too rich on Bank 1. The ECU has detected that it needs to subtract fuel to maintain the correct air/fuel ratio. Too much fuel or too little air is reaching the combustion chamber.
? Common Causes
- Dirty air filter restricting airflow
- Leaking fuel injector
- Faulty MAF sensor reading high
- High fuel pressure (stuck regulator)
- Faulty O2 sensor
- Stuck-closed purge valve
! Symptoms
- Check engine light
- Poor fuel economy
- Black exhaust smoke
- Sulphur/rotten-egg smell
- Spark plug fouling
- Rough idle
# Diagnostic Steps
- Check long-term fuel trim (LTFT < -15% indicates rich)
- Inspect and replace air filter
- Check fuel pressure (should drop with vacuum applied to regulator)
- Test MAF sensor readings
- Inspect injectors for leaking
- Test EVAP purge valve
⚙ Typical Fix
Replace dirty air filter, repair leaking injector, fix fuel pressure regulator, or replace faulty MAF sensor.
Parts Needed
Air filter, Fuel injector, Fuel pressure regulator, MAF sensor
Estimated Cost
$50-$500
↔ Related Codes
Q Frequently Asked Questions
Yes, but fuel is being wasted and the catalytic converter is being stressed. Running rich fouls spark plugs and can melt the catalyst over time.
Air filter: $15-$30. Fuel injector: $80-$200 each. Fuel pressure regulator: $50-$150.
☞ Learn More
- What is OBD-II? — Complete guide to your car's diagnostic system
- How to Read DTC Codes — Step-by-step with a Bluetooth scanner
- OBD-II Scanner Buying Guide — Find the best scanner for your car