P2271: O2 Sensor Signal Biased/Stuck Rich (Bank 1 Sensor 2)
The downstream oxygen sensor on bank 1 is stuck reading rich and does not respond when the mixture goes lean.
What P2271 means
P2271 is the stuck rich partner of P2270. The downstream oxygen sensor on bank 1 keeps reporting low oxygen, or rich, even when the computer leans out the mixture during its test. RepairPal lists a clogged air filter, oxygen sensor failure, computer failure and wiring problems as causes. If the engine really is running rich, the sensor is only telling the truth, so shops check fuel trims and the upstream sensor first. If the mixture is fine, the downstream sensor may be contaminated by fuel, oil or coolant, or simply worn out. An electrical problem where the signal wire touches the heater power wire in a damaged harness can also pin the reading high. Because the downstream sensor mostly monitors the converter, drivers often notice nothing, but the car can fail an emissions test. Testing usually includes forcing the engine lean with a vacuum leak or scan tool command and watching whether the sensor voltage drops below about 0.2 volts. If not, it is replaced.
Common symptoms
- Check engine light on
- Usually no change in driveability
- Possible fuel smell
- Failed emissions test
- Increased fuel consumption if the engine is truly rich
Most common causes
- Failing downstream oxygen sensor
- Engine running rich from another problem
- Clogged air filter
- Sensor contaminated by oil, coolant or fuel
- Signal wire shorted to the heater circuit
- Engine computer fault (rare)
What to check yourself first
Replace a dirty air filter. Check for rich codes like P0172. Inspect the downstream sensor wiring for melted spots where wires could touch.
Typical fixes and what they cost
| Fix | Typical cost | Note |
|---|---|---|
| Replace oxygen sensor | $440-$523 | RepairPal lists oxygen sensor replacement as the common repair. |
| Electrical diagnosis to find a wiring or connector fault | $122-$179 | For a signal wire shorted to the heater circuit. |
| Check engine light diagnosis at a shop | $125-$183 | Checks for an engine rich condition before replacing the sensor. |
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
- P2270 O2 Sensor Signal Biased/Stuck Lean (Bank 1 Sensor 2)
- P0138 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
- P2097 Post Catalyst Fuel Trim System Too Rich (Bank 1)
- P0420 Catalyst System Efficiency Below Threshold (Bank 1)
- P0172 System Too Rich (Bank 1)