CodelampTwo codes together
P0133 and P0171 together: a slow oxygen sensor and a lean mixture
P0133
- In plain words
- The front oxygen sensor reacts too slowly to changes in the exhaust.
- Standard name
- O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
- System
- Catalytic converter and oxygen sensors
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
P0133 says the front oxygen sensor on bank 1 reacts too slowly. P0171 says bank 1 is lean. Either the engine really is lean and the sensor is aging too, or the sensor is misreading a normal mixture. A shop tells them apart with live data.
Likely causes, most common first
- A real lean condition. A vacuum leak or low fuel pressure. The sensor may be reporting it correctly.
- An aging front oxygen sensor. Old sensors slow down, and a slow one can confuse the fuel correction.
- An exhaust leak near the front sensor. Outside air makes the exhaust look lean and slows the reading.
- Contamination of the sensor. Coolant or oil in the exhaust can coat it.
What you can note yourself
- Note the car's mileage and whether the oxygen sensors were ever replaced.
- Note any exhaust ticking noise when the engine is cold.
What to tell the shop
- Both codes, and the sensor's age if you know it.
- Ask them to confirm the lean condition with live data before replacing the sensor.
This page gives no verdict on whether a car is safe to drive; a mechanic can.