OBD-II trouble code
P2271: O2 Sensor Signal Stuck Rich (Bank 1, Sensor 2)
The downstream oxygen sensor on Bank 1 — behind the catalytic converter — is reporting a signal stuck toward rich. This is mainly an emissions and catalyst-monitoring code; the car almost always drives normally.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $400
- DIY difficulty
- Intermediate DIY
What does P2271 mean?
The downstream oxygen (O2) sensor on Bank 1 sits behind the catalytic converter and monitors catalyst efficiency rather than doing primary fuel control. On a healthy system its signal is fairly steady and mid-range because the catalyst buffers the oxygen swings the upstream sensor sees.
P2271 sets when the Bank 1 downstream sensor signal stays biased or stuck toward the rich side — a high, flat voltage above roughly 0.8 volts — instead of tracking the way the ECM expects. It's a rationality fault: the circuit is intact, but the reading is stuck and no longer plausible for the operating conditions.
Because this sensor does little fuel trimming on most vehicles, P2271 rarely affects driveability. The usual cause is an aged or contaminated downstream sensor stuck high. Contamination from oil or coolant burning in the exhaust, or a sensor soaked by a prior rich-running event, can bias it rich. Less often the signal is accurate and the engine or catalyst really is running rich — in which case you'll typically see upstream fuel-trim or catalyst codes alongside it. "Bank 1" is the side containing cylinder 1; "Sensor 2" is the downstream sensor.
Common causes
- Aged or contaminated downstream Bank 1 oxygen sensor stuck high (most common)
- Oil or coolant contamination in the exhaust coating the sensor
- Residual rich-running event that saturated the sensor
- Damaged, chafed, or corroded downstream sensor wiring or connector
- Sensor heater fault slowing response
- Genuine rich exhaust from an upstream fueling problem (usually with other codes)
Symptoms
- Check engine light is on
- Almost never any driveability symptom
- Failed emissions inspection
- Possible companion fuel-trim or catalyst codes
- Slight fuel smell only if the engine is genuinely rich
Diagnostic steps
- 1.Scan for all codes. Companion upstream rich codes (P0172/P0175) or a catalyst code point to a real rich condition rather than just the sensor.
- 2.Monitor Bank 1 Sensor 2 voltage live. It should be relatively steady mid-range; with P2271 it sits high and flat.
- 3.Inspect the sensor and its wiring for oil or coolant contamination and connector corrosion.
- 4.Confirm the engine is not genuinely running rich by checking upstream sensor behavior and fuel trims.
- 5.Check the sensor heater circuit if the signal is slow to respond.
- 6.If the engine fueling is correct and wiring is good, replace the downstream Bank 1 sensor.
Repair cost
$150 – $400
Downstream oxygen sensor replacement runs about $150 to $400 in parts and labor, with the sensor itself typically $40 to $150. If the code turns out to be caused by a genuinely rich engine, the cost tracks that underlying repair (leaking injector, high fuel pressure, or contaminated sensor from an internal leak).
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with oxygen sensor replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.