System Too Rich (Bank 1)

What it means

The engine computer is removing more fuel than normal to make up for too much fuel or too little air, and has reached its maximum correction on the bank containing cylinder 1. A rich mixture raises fuel use and, if it continues, can foul the spark plugs and overheat the catalytic converter.

Related codes: P0175P0171P0101P0132P0420P0300

Symptoms5

• Check engine light on.
• Clearly higher fuel consumption.
• Strong fuel smell or black smoke from the exhaust.
• Uneven idle or hard starting when hot.
• Sooty, black spark plugs.

Possible causes7

• A leaking injector or one that does not close properly.
• High fuel pressure from a failed pressure regulator or a blocked return line.
• MAF sensor over-reading, or a wrong reading from the manifold absolute pressure (MAP) sensor.
• Coolant temperature sensor falsely reading cold, so the computer adds fuel.
• EVAP purge valve stuck open, drawing in extra fuel vapor.
• Fuel diluting the engine oil, with vapors entering through the PCV system.
• Severely clogged air filter (less common on modern cars).

How to diagnose6

1. Check the fuel trims; large negative values confirm the computer is pulling fuel because the mixture is rich.
2. Measure fuel pressure running and after shutdown; a fast pressure drop after shutdown may indicate a leaking injector.
3. Compare the coolant temperature reading with actual engine temperature, and the MAF reading with the expected value.
4. Briefly pinch the purge valve hose or test the valve, and see whether the fuel trims improve.
5. Smell the engine oil; a fuel smell means it is diluted with fuel.
6. If P0175 is also stored, the cause is shared by both banks, such as fuel pressure or the airflow sensor.

How to fix6

1. Replace a clogged air filter and clean the MAF sensor.
2. Replace the EVAP purge valve if it is stuck open.
3. Replace the coolant temperature sensor if its reading is wrong.
4. Repair the fuel pressure regulator or return line if pressure is high.
5. Clean the injectors or replace a leaking one, and change the oil if it is diluted with fuel.
6. Clear the codes and confirm the fuel trims return to normal.

An air filter, MAF cleaning or purge valve are low-cost fixes; the cost rises when injectors or a pressure regulator are needed.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

Frequently asked questions