P2270: O2 Sensor Signal Biased/Stuck Lean (Bank 1 Sensor 2)
The downstream oxygen sensor on bank 1, after the catalytic converter, is stuck reading lean.
What P2270 means
P2270 applies to the downstream oxygen sensor on bank 1. During a test, the engine computer drives the mixture rich and lean and expects the downstream sensor to eventually follow. RepairPal explains that if it does not see the correct fluctuations, the computer sets this code, and lists oxygen sensor failure, a vacuum leak, wiring problems and computer failure as causes. Stuck lean means the sensor keeps reporting high oxygen even when the mixture goes rich. A sensor that has aged or been contaminated is a common cause, but an exhaust leak just ahead of or around the sensor is also very common because it lets outside air reach the tip. KBB notes these codes usually do not affect driving but will usually cause an emissions test failure. Shops check for exhaust leaks with the engine running, force the engine rich and watch whether the sensor rises above about 0.8 volts, and check the wiring before replacing the sensor.
Common symptoms
- Check engine light on
- Usually no change in driveability
- Failed emissions test
- Slightly higher fuel consumption on some cars
- May appear with P0420 or P2096
Most common causes
- Failing downstream oxygen sensor
- Exhaust leak near the sensor
- Vacuum leak
- Wiring problem
- Engine computer fault (rare)
What to check yourself first
Check the exhaust between the converter and the downstream sensor for rust holes or loose clamps. Inspect the sensor connector and wiring for road damage.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | RepairPal lists oxygen sensor replacement as the common repair. |
| Exhaust leak test | $61-$90 | Finds exhaust leaks near the sensor. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For wiring faults under the car. |
Typical US price ranges for parts and labor, not quotes. Your shop should confirm the cause with a diagnostic before replacing parts.
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Related codes
- P2271 O2 Sensor Signal Biased/Stuck Rich (Bank 1 Sensor 2)
- P0137 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
- P2096 Post Catalyst Fuel Trim System Too Lean (Bank 1)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0136 O2 Sensor Circuit (Bank 1 Sensor 2)