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Severity: Low Code type: Generic (all makes)

P2271 Code: O2 Sensor Signal Biased/Stuck Rich (Bank 1 Sensor 2)

The sensor after the converter stays rich even while coasting with the fuel cut off, when it should read lean.

At a glance

Severity: 2/5 – Low2/5

Severity: Low

Meaning
O2 Sensor Signal Biased/Stuck Rich (Bank 1 Sensor 2)
System
Fuel & Air Metering · Powertrain
Safe to drive?
Yes, short term
Typical repair cost
$90–$1,400
DIY difficulty
Easy

When you lift off the throttle at speed, the PCM shuts the injectors off completely. Pure air then flows through the engine, and within a few seconds the downstream sensor behind the catalytic converter should fall to a clearly lean reading. P2271 means Bank 1 Sensor 2 stayed on the rich side during that window, with a voltage that is within its normal range but in the wrong place. Unlike P0138, which usually points to an electrical short pushing the voltage too high, P2271 often has a real source behind it: something is still feeding fuel or oil into the engine with the injectors off. A purge valve that stays open under the strong vacuum of deceleration, an injector that drips, or oil pulled past worn valve seals can all keep the rear sensor rich. A contaminated or tired sensor is the other half of the picture.

Can I keep driving? Drivable; the engine usually runs normally. If the cause turns out to be fuel or oil entering during deceleration, the converter will suffer over time, so don't let it ride for months.

How to read P2271

How to read P2271P12223714
  1. P Powertrain
  2. 2 Generic (SAE)
  3. 2 Fuel & air (injector circuit)
  4. 71 Specific fault

Symptoms

  • Check engine light on
  • Rear O2 reading staying around 0.6–0.8 V even while coasting
  • Puff of blue smoke after a long downhill coast (oil-related cause)
  • Fuel smell after the engine is switched off (leaking injector or purge)
  • Later catalyst efficiency code on some vehicles

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Downstream sensor contaminated or aged, unable to drop to lean 28%
  2. EVAP purge valve leaking and feeding vapor during deceleration vacuum 16%
  3. Dripping injector or high fuel pressure adding fuel while injectors are off 16%
  4. Wiring fault: signal wire partly shorted to heater feed, poor sensor ground 14%
  5. Oil drawn in on deceleration past worn valve guides or seals, or a leaking turbo seal 10%
  6. Fuel-diluted engine oil venting vapor through the PCV system 8%
  7. Wrong or non-OE replacement sensor 8%

Diagnosis step by step

Diagnostic flow: P22711Use the front sensor as a witness2Take purge out of the loop3Check for leaking injectors and fuelpressure4Look for oil being pulled in5Test the circuit

  1. Use the front sensor as a witness

    Log both sensors while coasting in gear from about 45 mph. Confirm fuel cut (injector pulse width near 0 ms or fuel status 'open loop decel'). If the front sensor goes fully lean but the rear stays above 0.5 V, look at the rear sensor and its wiring; if the front stays rich too, fuel or oil is still getting in.

    Tools: OBD-II scanner

  2. Take purge out of the loop

    Clamp or disconnect and cap the purge line at the intake and repeat the coast-down. If the rear sensor now drops lean normally, replace the purge valve; a healthy normally closed valve also holds vacuum on the bench with no power.

    Tools: Pliers, Vacuum gauge

  3. Check for leaking injectors and fuel pressure

    Watch fuel pressure hold after shutdown; a steady drop without an external leak points to injectors or the regulator. Check that rail pressure matches spec at idle.

    Tools: Fuel pressure gauge

  4. Look for oil being pulled in

    Have a helper watch the tailpipe as you accelerate after a long closed-throttle coast. A puff of blue smoke points to valve guides or seals, or on turbo engines to a leaking turbo seal; check oil consumption between changes.

    Tools: Flashlight

  5. Test the circuit

    With the sensor unplugged and key on, the signal circuit to the PCM should carry only a small bias voltage, never heater voltage. Check the sensor ground for voltage drop, then compare readings with a known-good sensor if one is available.

    Tools: Multimeter

Where the parts are

Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2) – P22711212345
Exhaust: catalytic converter and O2 sensors (Bank 1 / Bank 2)
  1. 1Upstream O2 sensor
  2. 2Catalytic converter
  3. 3Downstream O2 sensor
  4. 4Diesel particulate filter
  5. 5Turbocharger

Repair options & costs

RepairDIY?Estimated cost
Replace downstream O2 sensor Yes $120–$350
Replace EVAP purge valve Yes $90–$250
Replace leaking injector(s) No $250–$900
Replace valve stem seals No $500–$1,400

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Emissions / smog test impact

Will fail

A lit check engine light fails the OBD-II test. The rear sensor is also used for the catalyst monitor, so that monitor may stay not-ready until P2271 is fixed.

Frequently asked questions

What's the difference between P2271 and P0138?

P0138 means the voltage is above the circuit's normal range, which usually points to a short. P2271 means the signal is in a normal range but stays rich when it should be lean, so real sources of fuel or oil during deceleration are more likely.

How can the sensor read rich when no fuel is injected?

Something else is supplying fuel or hydrocarbons: an open purge valve, a dripping injector or oil pulled in by the high vacuum of deceleration. If none of those are present, the sensor itself is stuck.

Should I just replace the rear sensor?

Do the coast-down test with both sensors first. If the front sensor also stays rich, a new rear sensor won't fix anything; if the front goes lean and the rear doesn't, the sensor or its wiring is the likely fault.

Sources