P2271
O2 Sensor Signal Biased/Stuck Rich Bank 1 Sensor 2
The engine computer sees the rear oxygen sensor on bank 1 staying too rich or not reacting the way it should. The rear sensor does not usually control fuel mixture directly, but its signal still has to make sense. The problem may be a biased sensor, wiring fault, contamination, catalyst problem, or a real rich-running engine condition.
- SEV
- 3/5
- DRIVE
- CAUTION
- DIY
- $20-$150
- SHOP
- $150-$500
Quick answer
AI-CITATION READYWhat it means
Can you drive with it?
Most common causes
- Faulty Bank 1 Sensor 2 oxygen sensor
- Damaged, corroded, or loose sensor wiring or connector
- Sensor contamination from coolant, silicone, oil, or fuel additives
Typical repair cost
DIY usually runs $20-$150. Typical shop repair lands around $150-$500, depending on the root cause.
01 / Definition
P2271 means the PCM detected that the Bank 1 Sensor 2 oxygen sensor signal is biased high or stuck rich for too long. Bank 1 Sensor 2 is the downstream oxygen sensor after the catalytic converter. The PCM expects this sensor to move more slowly than the upstream sensor, but it still has to respond to normal exhaust changes and catalyst storage behavior. If the signal stays rich, the fault may be a biased rear sensor, short to voltage, connector contamination, catalyst storage problem, or an actual rich-running engine condition that keeps the exhaust loaded with fuel.
02 / Drive status
With caution. You can usually drive short distances, but the vehicle should be checked soon. This code often points to an emissions-monitoring problem more than a severe drivability fault, but if the engine is truly running rich, fuel economy can drop and catalytic converter damage can happen over time.
03 / Symptoms
- Check engine light
- Poor fuel economy
- Possible fuel smell
- Failed emissions test
- Stored rich or oxygen sensor codes
- No noticeable drivability problem in many vehicles
04 / Causes
| 1 | Faulty Bank 1 Sensor 2 oxygen sensor | high |
|---|---|---|
| 2 | Damaged, corroded, or loose sensor wiring or connector | high |
| 3 | Sensor contamination from coolant, silicone, oil, or fuel additives | medium |
| 4 | Engine running rich because of leaking injectors, high fuel pressure, or airflow problems | medium |
| 5 | Exhaust restriction or catalyst issue affecting downstream sensor behavior | medium |
| 6 | PCM software issue or rare PCM fault | low |
05 / Diagnostic sequence
- 01Scan for all stored and pending codes and review freeze-frame data before clearing anything.
- 02Scan for related rich, fuel trim, misfire, catalyst, and oxygen sensor heater codes.
- 03Inspect the Bank 1 Sensor 2 wiring, connector, and harness for heat damage, corrosion, looseness, or oil contamination.
- 04Review live data from Bank 1 Sensor 1 and Bank 1 Sensor 2 to compare sensor behavior after the engine is fully warm.
- 05Check whether the rear sensor is stuck high all the time or only during the same conditions shown in freeze-frame data.
- 06Check short-term and long-term fuel trims to see whether the engine is actually running rich.
- 07Inspect for leaking injectors, excessive fuel pressure, contaminated MAF readings, or other causes of rich operation.
- 08Inspect the rear sensor tip and connector for contamination from coolant, silicone sealant, or oil if accessible.
- 09Check for short-to-voltage faults, poor grounds, or a biased signal circuit if the rear sensor stays high with key on or during harness movement.
- 10Check for exhaust restriction or downstream catalyst storage issues if the rear sensor stays biased.
- 11Test the sensor circuit and replace the oxygen sensor if it fails output or response checks.
- 12If the problem returns, verify PCM updates and perform advanced fuel-system diagnosis.
06 / Repairs
| 1 | Repair damaged wiring or connector problems near Bank 1 Sensor 2 | $20-$150 |
|---|---|---|
| 2 | Replace the Bank 1 Sensor 2 oxygen sensor if it is biased, slow, or not responding correctly | $50-$200 |
| 3 | Clean up contamination sources or repair coolant, oil, or sealant leaks that foul the rear oxygen sensor | $20-$400 |
| 4 | Repair rich-running causes such as leaking injectors, fuel pressure faults, or airflow measurement problems | $80-$900 |
| 5 | Correct exhaust restriction or catalyst issues affecting downstream sensor behavior | $100-$1,200 |
| 6 | Perform PCM software updates or advanced electrical diagnosis if the code persists | $0-$200 |
07 / Related codes
08 / FAQ
What does Bank 1 Sensor 2 mean?
It is the downstream oxygen sensor on the side of the engine with cylinder 1, located after the catalytic converter.
Can a rich-running engine cause P2271?
Yes. If the engine is truly running rich, the downstream sensor can stay biased rich and trigger the code.
Does P2271 mean the rear sensor controls fuel mixture?
Usually no. Bank 1 Sensor 2 is mainly there to monitor catalyst and exhaust behavior, but a bad signal can still set the code and confuse catalyst monitoring.
Will replacing the oxygen sensor always fix P2271?
No. You should also check for rich-running engine conditions, wiring faults, and exhaust or catalyst problems.
09 / Source and method
- DATA BASIS
- OBD-II REFERENCE + OBD2.HELP
- METHOD
- STATIC VALIDATION
- SAFETY
- INFORMATIONAL
This page combines OBD-II diagnostic reference data with OBD2.help generated diagnostic guidance for code meaning, likely causes, and repair direction.
Publishing uses deterministic schema and build validation, plus manual spot checks on representative pages before release.
Safety-critical diagnosis and repairs should be confirmed with a qualified mechanic, especially when the vehicle is misfiring, overheating, or losing power.