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P codepowertrainMedium severity

P2277O2 Sensor Signal Biased/Stuck Rich Bank 2 Sensor 3

The third oxygen sensor on Bank 2 is stuck reporting a rich exhaust condition and will not swing lean even when conditions should drive it low.

Definition

The PCM evaluates the Bank 2 Sensor 3 oxygen sensor — the furthest downstream sensor on that bank — by confirming its voltage drops during lean events such as deceleration fuel cutoff. When the signal stays pinned high near 0.8-0.9 V and refuses to fall, the PCM sets P2277 for a signal biased or stuck rich. Driveability is usually unaffected, but the MIL illuminates, rear-catalyst monitoring is suspended, and the vehicle will fail an OBD emissions inspection; a genuinely rich exhaust can also overheat the converters over time.

Common causes

  • Contaminated oxygen sensor (oil ash, coolant, or fuel additive deposits) biased rich
  • Signal wire shorted to another voltage source or degraded sensor internals
  • Genuine rich condition on Bank 2 (leaking injector, high fuel pressure, misfire dumping fuel)
  • Silicone contamination from non-sensor-safe sealants
  • Water-contaminated connector creating a voltage offset

Common misdiagnoses

  • Rear catalytic converter replaced for a sensor bias problem
  • The wrong oxygen sensor on Bank 2 replaced (Sensor 1 or 2 instead of 3)
  • Fuel pressure regulator or injectors replaced without confirming an actual rich condition
  • PCM replaced for what is sensor contamination

Troubleshooting steps

  1. 1.Check fuel trims and companion codes first

    Rich codes (P0172/P0175) or strongly negative Bank 2 trims indicate a real rich condition to fix before condemning the sensor.

  2. 2.Watch B2S3 voltage during deceleration fuel cutoff

    Coast down from about 50 mph in gear with your foot off the throttle; the sensor should drop below 0.2 V — a reading held above 0.6 V is stuck rich.

  3. 3.Inspect the sensor tip when removed

    White or tan ash suggests oil or coolant contamination and shiny black soot suggests a rich mixture; either indicates an upstream engine problem plus a compromised sensor.

  4. 4.Test the signal wiring for shorts to voltage

    With the sensor unplugged and key on, the signal circuit should carry only bias voltage; higher readings mean a harness short is offsetting the signal.

  5. 5.Replace the sensor and correct any root cause

    Install an OEM-equivalent sensor, repair whatever contaminated the original, clear codes, and verify the signal now swings with mixture changes.

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