OBD-II trouble code
P2271: O2 Sensor Signal Stuck Rich (Bank 1, Sensor 2)
The post-catalyst sensor on Bank 1 is pinned high and no longer plausible. Before anyone quotes a catalytic converter off the back of it, understand that this exact fault is the most common reason a healthy converter gets condemned.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $450
- DIY difficulty
- Intermediate DIY
What does P2271 mean?
P2271 is a plausibility judgement rather than a circuit fault. The wiring is intact and the controller is receiving data — it simply does not believe it. The post-catalyst sensor on Bank 1 is sitting high, typically above about 0.8 volts, and staying there through operating conditions that should have moved it.
The reason this matters far more than its severity rating suggests comes down to how catalyst efficiency is judged. The controller has no direct way to measure what a converter is doing. It infers it, by comparing the busy switching of the sensor ahead of the converter against the calm of the sensor behind it. A working converter absorbs and releases oxygen, which smooths the downstream signal out. So the monitor is really asking one question: is the rear sensor much quieter than the front one?
A rear sensor stuck high answers yes in the most emphatic way possible — and it answers yes whether the converter is perfect or destroyed. That is the trap. A stuck-rich rear sensor does not merely accompany a catalyst efficiency code; it is capable of manufacturing one out of nothing. When P2271 and a catalyst code are stored together, the overwhelmingly likely reading is one failed sensor producing two complaints, not a coincidence of a dying sensor and a dying converter. The price difference between those two interpretations is roughly tenfold.
The sequence that follows from this is not optional. Repair the sensor, clear both codes, then drive enough complete cycles for the catalyst monitor to actually run before anyone forms an opinion about the converter. Acting on a converter verdict that was produced by a lying sensor is the single most expensive mistake available on this code.
The less common alternative is that the sensor is telling the truth and the exhaust really is rich — but a genuinely rich engine announces itself upstream first, with fuel trim codes and a front sensor that is also biased. If those are absent, the rear sensor is the story.
Common causes
- Sensor element contaminated and no longer able to switch — the dominant cause by a wide margin
- Silicone poisoning from the wrong RTV sealant used during a previous repair, which leaves a white glassy deposit
- Coolant reaching the exhaust through a failing head gasket or cracked head, leaving a chalky white residue
- Oil consumption past valve stem seals or rings coating the element in soot and ash
- Sensor saturated by an extended rich-running event and never recovered
- Genuinely rich exhaust from a fuelling fault — credible only when upstream fuel trim codes are stored as well
Symptoms
- Check engine light on
- Catalyst efficiency code frequently stored alongside it
- Live data shows the Bank 1 post-catalyst sensor flat and high regardless of throttle or load
- Emissions inspection failed
- Almost never any change in how the vehicle drives
- Visible exhaust smoke or coolant loss only when contamination is the underlying cause
Diagnostic steps
- 1.Read every stored code first. Upstream rich codes such as P0172 or P0175 change the diagnosis completely — they mean the sensor may be honest and the engine is the problem.
- 2.Graph the post-catalyst sensor rather than reading a single value. Pinned high and unresponsive to a snap-throttle or a decel fuel-cut event confirms it is stuck rather than merely reporting a rich moment.
- 3.If a catalyst efficiency code is stored with this one, do not act on it yet. Treat the sensor as the prime suspect and re-evaluate the converter only after the sensor is repaired and the monitor has re-run.
- 4.Remove the sensor and inspect the tip. The deposit identifies the poison: white and glassy points at silicone, chalky white at coolant, black and oily at oil consumption, and a clean grey-brown tip means simple age.
- 5.If the tip shows contamination, find and fix the source before fitting a new sensor. A replacement installed into the same exhaust stream will be poisoned exactly the same way, usually within months.
- 6.Check the sensor heater circuit and connector, then confirm upstream fuel trims are within normal range before concluding the sensor is at fault.
Repair cost
$150 – $450
Post-catalyst sensor replacement typically runs $150 to $450 fitted, the sensor itself being $40 to $150 of that. The number worth keeping in view is the one this code can wrongly trigger: acting on a catalyst efficiency code caused by this sensor means a converter at $900 to $2,500 or more. If contamination is the cause, the real repair is whatever is leaking oil or coolant into the exhaust, and the sensor is a consequence rather than the job.
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.