P0172 — System Too Rich (Bank 1)
Long-term fuel trim on bank 1 is at the lower limit — the ECU is removing fuel from baseline but mixture remains too rich. Indicates excess fuel delivery, restricted air flow, or a faulty oxygen sensor reporting falsely lean.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Fuel pressure regulator
- Fuel injector
- Air filter
- Oxygen sensor upstream
- Mass air flow sensor
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Fuel pressure regulator stuck closed
- Leaking fuel injector
- Severely blocked air filter
- Oxygen sensor reporting incorrectly
Possible symptoms
- Check engine light on
- Increased fuel consumption
- Black smoke from exhaust on acceleration
- Smell of unburned fuel, fouled spark plugs
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Related codes
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.