P0172: System Too Rich — Bank 1
What to do
Expect poor economy, a fuel smell, possibly black smoke and sooty plugs. Worth prompt attention: prolonged rich running washes oil from the cylinder walls and overheats the catalytic converter. If symptoms are worst immediately after filling up, mention that — it points at the evaporative system.
What this code means
The module has been removing fuel to compensate for a persistently rich mixture on bank 1, and has reached the limit of what it can subtract. Either more fuel is entering than was commanded, or less air than was measured. Leaking injectors and excessive fuel pressure account for most of the first; a restricted air filter or an over-reporting airflow sensor for the second. A saturated evaporative charcoal canister is the cause people forget — it dumps stored fuel vapour into the intake and shows up most strongly just after refuelling.
Likely causes
Start here
- Leaking fuel injector · moderate
- Excessive fuel pressure · moderate
- Restricted air filter · easy
- Contaminated or failing mass air flow sensor · easy
- Saturated charcoal canister · moderate
Less common
- Faulty engine coolant temperature sensor · easy
- Faulty oxygen sensor · easy
- Blocked fuel return line · moderate
Symptoms
Components involved
Information, not instruction
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Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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