P0172 — System Too Rich (Bank 1)

P0172 means the engine computer is pulling fuel out because the exhaust reads too rich. Common causes are a dirty MAF sensor, leaking injectors, or high fuel pressure.

Cost range: $50–$900. Severity: medium. Safe to drive? Short term yes, but a persistently rich mixture can damage the catalytic converter and wash cylinder walls.

Most common causes of P0172

1. Dirty or failing MAF sensor — Over-reports airflow so the ECM injects too much fuel. ($20–$400)
2. Leaking fuel injector — Drips fuel into the cylinder outside the injection event. ($200–$700)
3. Faulty fuel pressure regulator — Holds rail pressure too high. ($150–$450)
4. Restricted air filter — Cheapest thing to rule out first. ($20–$60)
5. Failing upstream O2 sensor — Lazy sensor biases the fuel trim. ($150–$400)

Symptoms drivers report with P0172

• Fuel smell
• Black smoke
• Poor fuel economy
• Rough idle
• Fouled spark plugs