P2271: O2 Sensor Signal Biased/Stuck Rich (Bank 1, Sensor 2)
Key takeaways
- P2271 means the downstream oxygen sensor on bank 1, the one behind the catalytic converter, didn't drop to a lean reading when the computer expected it to.
- The engine usually runs the same, so it's safe to drive, but the car will fail an OBD emissions inspection.
- An aged rear sensor is the most common answer, but a wiring short or a truly rich engine can set it too, so check before buying a $150–$400 sensor.
- On 2005–2008 Honda Accord and 2007–2008 Acura TSX, Honda says the likely cause is the engine computer misreading the front air-fuel sensor, not a bad rear sensor.
What P2271 means
The oxygen sensor behind your catalytic converter on bank 1 keeps reading rich when the computer expects it to go lean. Scanners call it O2 Sensor Signal Biased/Stuck Rich (Bank 1, Sensor 2). Sensor 2 is the downstream sensor, and its main job is to tell the computer whether the converter is working. To test it, the computer watches for moments when the exhaust should be lean, such as when you lift off the gas and fuel is cut. A healthy rear sensor drops to a low voltage then. If it stays high, P2271 sets.
Bank 1 is the side of the engine with cylinder 1; on a four-cylinder it’s the only bank.
Is it safe to drive?
Yes, in most cases. On most cars the rear sensor has little say over fueling, so the engine drives the same. You’ll fail an OBD emissions inspection with the light on, and the converter test can’t run properly until this is fixed.
What changes the answer: a rough-running engine, a strong fuel smell, black smoke, or a flashing check engine light. Those mean the engine really may be rich or misfiring, and unburned fuel can overheat the catalytic converter. Get that checked soon.
Causes, ranked
| Cause | How you confirm it | Typical cost (US independent shop, 2026) |
|---|---|---|
| Aged or contaminated downstream sensor | Signal stays high on a hard deceleration while the front sensor goes lean; high-mileage original sensor | $150–$400 |
| Signal wire shorted to heater or power wire, water in the connector | Melted or chafed harness under the car, corrosion in the plug; heater code P0141 set too | $100–$250 |
| Front air-fuel sensor misread or failing (some Honda and Acura) | Bulletin matches your car; front sensor data looks wrong | $150–$400 for the front sensor, or a software update |
| Engine really running rich (leaking injector, purge valve, high fuel pressure) | Front sensor and fuel trims also show rich; P0172 stored | $120–$350 purge valve; $250–$600 per injector |
| Engine computer software (some makes) | A bulletin for your exact model and year names P2271 | $100–$200, often $0 under warranty |
Trucks and sensors rusted into the pipe land at the top of these ranges.
What to check first
- Read all the codes. With rich or misfire codes, fix those first and the rear reading may settle. With heater codes for the same sensor, start with the fuse and wiring.
- Look for a bulletin for your model and year before buying a part.
- Inspect the harness under the car. Look for wires touching the exhaust, a melted clip, or water and green corrosion in the connector.
- Watch live data on a test drive. A shop snaps the throttle shut at speed and watches whether the rear sensor drops. If the front sensor and trims show a truly rich engine, the rear sensor is fine.
- Replace the rear sensor last, then clear the code and drive a normal mixed trip with a few decelerations to confirm.
The bulletin pattern is mostly software and signal interpretation. Honda and Acura tech-line articles for the 2005–2008 Accord and 2007–2008 TSX say the computer is likely misreading the front air-fuel sensor, which makes it think the rear sensor is stuck rich. Ram has an engine computer reflash for 2011 2500 pickups with the 6.7L Cummins diesel, and Land Rover covers oxygen sensor codes on the 2016 Range Rover.
What it costs
The cheap outcome is a wiring or connector repair at $100–$250, or a software update, often free under warranty. The common outcome is a new downstream sensor at $150–$400. The expensive outcome is fixing a real rich condition upstream, up to $600 for an injector. A $100–$180 diagnosis with live data keeps you from replacing the rear sensor when the front one, or the engine, is the problem. Our O2 sensor guide covers the whole sensor family.
Related codes
P2271 often appears with P0138 (high voltage, same sensor) or P0141 (its heater). P2273 is the same fault on bank 2. If P0420 shows up too, the converter test may be reading bad data from this sensor, so fix P2271 first. 3 complaints filed with NHTSA mention P2271; those are owner reports, not confirmed causes.
What owners report to NHTSA
3 complaints filed with NHTSA mention P2271, 3 of them since 2015, across 3 different models. The component owners most often filed it under: engine.
Manufacturer service bulletins that name P2271
7 technical service bulletins and manufacturer communications filed with NHTSA mention P2271, from 4 makes. A bulletin means the manufacturer has documented a known cause or a fix procedure for that model. Examples:
- LAND ROVER RANGE ROVER (2016), bulletin 10105881: "ISSUE: THE CUSTOMER MAY OBSERVE A MIL IN THE INSTRUMENT CLUSTER RELATING TO THE ENGINE SYSTEM (CHECK ENGINE LIGHT) AND ONE OR MORE CODES STORED IN THE PCM RELATING TO HEATED OXYGEN SENSOR (HO2S) IN THE EXHAUST SYSTEM. T…
- DODGE RAM 2500 (2011), bulletin 10128924: Flash: Diagnostic And System Improvements This bulletin involves selectively erasing and reprogramming the Engine Control Module (ECM) with new software. Pickup trucks equipped with a 6.7L Cummins diesel have software im…
- ACURA TSX (2007, 2008), bulletin 10142451: Tech line summary article - Is the MIL on with DTC P2271 (secondary HO2S [sensor 2] circuit signal stuck rich)? What's likely causing this is the PCM is misinterpreting the A/F sensor (sensor 1) signals, causing the PCM …
- HONDA ACCORD (2006, 2007, 2005, 2008), bulletin 10142447: Tech line summary article - Is the MIL on with DTC P2271 (secondary HO2S [sensor 2] circuit signal stuck rich)? What's likely causing this is the PCM is misinterpreting the A/F sensor (sensor 1) signals, causing the PCM …
Sources: NHTSA Office of Defects Investigation complaint database and manufacturer communications file, snapshots September 2026. A complaint or bulletin that mentions a code is a report about that vehicle, not a statement that the code has one cause.
Frequently asked questions
Can I drive with P2271?
Usually yes. The rear sensor mainly checks the catalytic converter, so the engine drives normally. If you smell fuel, see black smoke, or the light flashes, the engine may really be running rich, and that needs a look soon.
Where is bank 1 sensor 2?
It's the oxygen sensor after the catalytic converter on the side of the engine with cylinder 1. On a four-cylinder there's only one bank, so it's simply the rear sensor, usually reached from under the car.
Will a new rear O2 sensor fix P2271?
Often, on a high-mileage original sensor. But on some Hondas and Acuras the fault is in how the front sensor is read, and a shorted signal wire or a rich-running engine will set the same code on a new sensor. Check wiring, fuel trims and bulletins first.
Can P2271 damage the catalytic converter?
The sensor fault itself won't. If the rich reading is real and the engine runs rich for months, extra fuel can overheat and damage the converter.