CodelampCame back after a repair
P0171 came back after a new oxygen sensor
P0171
- In plain words
- The engine is getting too much air or too little fuel on bank 1.
- Standard name
- System Too Lean (Bank 1)
- System
- Fuel and air mixture
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
The front oxygen sensor reports a lean mixture; it rarely causes one. Replacing it fixes P0171 only when the old sensor was misreading. A new sensor that reports the same thing usually means the engine really is lean.
Likely causes, most common first
- A vacuum leak. The most common cause of a real lean condition.
- A dirty or failing mass airflow sensor. It under-reads the air, so the engine gets too little fuel.
- Low fuel pressure. A weak pump or clogged filter.
- An exhaust leak near the sensor. Outside air makes the exhaust look lean.
What you can note yourself
- Note which sensor was replaced and how soon the code came back.
- Note when the car runs worst: idle, cold or highway.
What to tell the shop
- Which sensor was replaced, and when.
- Ask them to confirm the lean condition with fuel trim data and a smoke test.
This page gives no verdict on whether a car is safe to drive; a mechanic can.