P2271
- Powertrain
- Manufacturer-specific
- Emissions
Diagnose soon
By itself this is a monitor and emissions failure, not a driveability hazard. It becomes more serious if the cause is a genuinely rich-running engine, because raw fuel damages the catalytic converter and dilutes engine oil. Check fuel trims before deciding which situation you are in.
- Safe to drive
- Generally drivable short term. If fuel trims are strongly negative or you smell fuel, get it addressed promptly to protect the converter.
O2 Sensor Signal Stuck Rich (Bank 1, Sensor 2)
The rear (downstream) oxygen sensor on bank 1 is reporting a rich exhaust and will not move lean when the computer forces the mixture lean. Either the exhaust genuinely is rich, or the sensor is no longer reporting honestly.
The PCM runs an intrusive rationality test on the post-catalyst sensor: it commands a lean fuel bias, or uses deceleration fuel cut, and expects the sensor signal to fall toward the lean end of its range. P2271 sets when the signal stays biased toward rich despite that command, or sits pinned rich across a defined time window. This is a signal-plausibility (rationality) fault on the sensor's reported value — not a circuit-integrity fault, not a catalyst efficiency fault, and not a fuel trim fault. It can set with the wiring perfectly intact.
How the car detected itThe module compares post-catalyst sensor output against what it knows the exhaust must contain under a controlled condition. During deceleration fuel cut there is no fuel being injected, so the exhaust is pure air and any working sensor must read lean. A conventional narrowband zirconia sensor sweeps roughly 0.1 to 0.9 volts; stuck rich means the signal sits high and flat instead of dropping. Some vehicles use a wideband sensor downstream, in which case the module evaluates the sensor's reported lambda (equivalence ratio), derived from its pumping current, rather than a switching voltage. The units and displayed values differ by make.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Generally drivable short term. If fuel trims are strongly negative or you smell fuel, get it addressed promptly to protect the converter.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- The vehicle will fail an emissions test and the catalyst monitor will not complete. If the underlying condition is real rich running, the converter can overheat and break down, fuel washes cylinder walls, and engine oil gets diluted with fuel.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Increased fuel consumption
- Possible rough idle or hesitation
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Downstream sensor failed stuck rich | Very common | — |
| 2 | Downstream sensor contaminated | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Long-term and short-term fuel trim on bank 1
If trims are strongly negative, the computer is already pulling fuel out to compensate. The engine really is rich and the sensor is telling the truth. Chasing the sensor then wastes the part.
Every other stored code, in the order they set
Misfire, MAF, fuel pressure, or injector codes stored alongside P2271 are often the underlying cause rather than a side effect. Fix upstream faults first and retest — this code frequently does not return.
Freeze frame data
Engine temperature, load, and rpm at the moment of the fault tell you whether this happens cold, hot, at idle, or at cruise, which narrows the list before you touch anything.
Engine oil level and coolant level
An engine consuming oil or coolant contaminates the sensor element. A sensor that is stuck rich because of silicone, phosphorus, or coolant contamination will do the same thing to its replacement.
The sensor connector and harness routing
A signal wire shorted to a voltage source — most often the heater B+ feed running in the same harness — drives the signal high and forces a false rich reading, and the fix is a $20 repair instead of a sensor. A short to ground does the opposite: it pulls the signal toward zero, which the module reads as lean, and that shows up as a stuck-lean code rather than this one. Downstream harnesses run near the exhaust and get heat damage.
Record freeze frame and the full code set before clearing anything
Tools Scan tool with freeze frame access
Note engine coolant temperature, rpm, load, and vehicle speed. Capture every code from every module, not just the powertrain ones. Do not clear codes yet; the freeze frame is the only snapshot of the failing condition you get.
ExpectedA written record of the conditions at the fault and every stored code, captured before anything is cleared
Read fuel trims at idle and at a steady 2,500 rpm
Tools Scan tool live data
Watch short-term and long-term trim on bank 1. Trims outside roughly plus or minus 10 percent are worth investigating on any OBD-II vehicle. Negative trims mean the computer is subtracting fuel to correct a rich condition.
ExpectedFuel trims within roughly plus or minus 10 percent if the mixture is genuinely correct
Graph both bank 1 oxygen sensors together
Tools Scan tool with graphing, or lab scope on the signal wire
The upstream sensor should switch actively. A healthy downstream narrowband sensor sits fairly steady in the mid-to-upper range behind a good converter but must still respond to large mixture changes. A downstream signal that is a flat line near the top of its range under all conditions is the fault the code describes.
ExpectedAn actively switching upstream sensor, and a downstream sensor that still responds to large mixture changes rather than sitting flat at the top of its range
Force a deceleration fuel cut and watch the downstream sensor
Tools Scan tool live data, safe road or dyno
Bring the vehicle to about 40 mph, then close the throttle fully and coast in gear until below about 20 mph. With injectors off, the sensor is seeing air and must swing lean. If it does not move, the sensor or its circuit is the problem. If it swings lean normally, look harder at the fuel control system.
ExpectedThe downstream signal falls toward the lean end of its range during a sustained deceleration. Because the converter stores oxygen, the post-cat sensor lags the upstream sensor noticeably — allow several seconds of continuous fuel cut before judging it. A signal that never moves at all across repeated decel events is the fault; a slow fall is normal.
Test the sensor circuit at the connector, not at the PCM
Tools Digital multimeter, factory wiring diagram, backprobe leads
With the connector mated and the engine running, backprobe the signal and signal-return wires to observe live signal voltage. Then, with the key off, unplug both the sensor and the module connector before making any resistance measurement: check the signal wire for continuity end to end, for a short to ground, and for a short to the heater feed or any other voltage source. Resistance readings taken with the connectors still mated read through the sensor element and the module's internal circuitry and are meaningless. Wire colors and pin assignments vary by make, so use the factory wiring diagram rather than guessing.
ExpectedA live signal that moves with mixture, and with both connectors unplugged, continuity end to end with no short to ground and no short to any voltage source
If fuel trims are negative, test the fuel side before condemning the sensor
Tools Fuel pressure gauge, scan tool, injector balance test equipment
Check fuel pressure against the vehicle specification, look for a leaking injector, a stuck-open purge valve, or a failing upstream sensor pulling the mixture rich. Fuel pressure specs vary widely by fuel system type and must come from the service data. Relieve fuel pressure before opening any fuel connection, keep an extinguisher within reach, and treat a pressurized rail as a fire hazard.
ExpectedFuel pressure matching the service-data figure for that fuel system, no injector leaking down, and the purge valve sealed when commanded closed
Inspect the sensor tip once removed
Tools Oxygen sensor socket, good light
White or gritty deposits point to coolant or silicone. Black soot points to real rich running. Oily glaze points to oil consumption. The tip tells you whether replacing the sensor alone will hold.
ExpectedLight grey to light tan deposits on a healthy sensor
Clear codes and complete a full drive cycle before declaring it repaired
Tools Scan tool with readiness monitor status
The oxygen sensor monitor will not run until its enabling conditions are met, which typically requires a cold start plus mixed driving. Confirm the monitor has actually run and passed rather than assuming a dark dashboard means success.
ExpectedBank 1 oxygen sensor monitor status shows complete and no codes return
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Repair a shorted, chafed, or heat-damaged downstream sensor harness or connector | With the sensor and module connectors unplugged, the signal wire measures shorted to a voltage source such as the heater feed, or the signal-return circuit measures open or high-resistance in a way that biases the signal high | $120–$450 | DIY |
| Replace the bank 1 downstream oxygen sensor | The sensor fails to swing lean across repeated sustained deceleration fuel cuts while fuel trims are normal and the circuit tests good | $150–$450 | DIY |
| Correct a genuine rich-running condition, such as a leaking injector, excessive fuel pressure, or a stuck-open purge valve | Negative fuel trims plus a specific failed test: injector leak-down, fuel pressure above specification, or purge valve flowing when commanded closed | $200–$900 | DIY |
| Repair the cause of oil or coolant consumption that is contaminating the sensor | Contaminated sensor tip together with documented oil or coolant loss and a located source such as valve seals, rings, or a head gasket | $400–$4,000 | Shop |
| Replace the catalytic converter | Only after the sensor and mixture check out, and a converter efficiency test or code independently shows the converter has failed. P2271 alone never justifies this. | $700–$2,500 | Shop |
Sensor prices swing hard between an OE part and an aftermarket one, and a sensor seized into a rusty bung can add an hour of labor or turn into an exhaust repair. Ranges are US independent-shop, parts plus labor.
- Replacing the downstream sensor while fuel trims sit strongly negative, which means the engine really is rich and the new sensor will report the same thing.
- Assuming any downstream oxygen sensor code means the catalytic converter has failed. P2271 is about the sensor signal, not converter efficiency.
- Hunting for a short to ground on a stuck-rich code. A grounded signal wire reads lean; only a short to a voltage source drives the signal falsely rich.
- Mixing up sensor 1 and sensor 2, or bank 1 and bank 2, and replacing a perfectly good sensor. Bank 1 contains cylinder number 1, and which physical side that is varies by engine.
- Using ordinary RTV silicone or non-sensor-safe thread compound near the exhaust or intake, which poisons the new sensor within days.
- Clearing the code and handing the car back before the monitor has run. The code is not fixed until the monitor completes and passes.
- Ignoring an engine that is quietly burning oil or coolant, which will contaminate every sensor you install.
- Oxygen Sensor Downstream
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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