P0171: system too lean
The engine is getting too much air relative to fuel. It is usually safe to drive in the short term, but have it checked soon, because running lean for long wears on the engine.
Can you keep driving?
Yes, in the short term. Do not leave it for months.
What the code means
The control unit is measuring more oxygen in the exhaust than it expects from the amount of fuel it is injecting.
Bank 1 is the side of the engine where cylinder 1 sits. Most four-cylinder engines have only one bank, so there you normally see just P0171.
Common causes
- A vacuum leak: a cracked or loose hose letting in unmetered air
- A dirty or failing mass airflow sensor
- A problem with the fuel supply
What to do now
- Look for cracked or loose hoses in the engine bay.
- Listen for a sucking or whistling sound at idle.
- Have the mass airflow sensor assessed before it is replaced.
What people often get wrong
Replacing the mass airflow sensor first, because it is easy to change. A vacuum leak gives exactly the same code and costs a length of hose.
When the car must go to a workshop
Running lean for a long time contributes to the wear that lean-running engines are prone to. Do not leave it for months.
Read next
- P0300: misfire on more than one cylinder
- Your check engine light isn't random
- P0135: the oxygen sensor heater circuit
This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.
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