OBD-II trouble code
P2275: O2 Sensor Signal Biased/Stuck Rich (Bank 1, Sensor 3)
The rearmost oxygen sensor on Bank 1 is biased high. A rich bias cannot be produced by an air leak, so the whole inspection order flips compared with its stuck-lean sibling.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $140 – $450
- DIY difficulty
- Intermediate DIY
Browse every code in P2200–P229F, or start from the full code library.
What does P2275 mean?
P2275 says the third sensor on Bank 1 is sitting high and refusing to move — reporting an oxygen-starved, fuel-rich exhaust stream at the very back of the system. The circuit is not open or shorted; the number is simply not credible for the conditions.
The direction of the bias is the most useful piece of information on this page. Leaks admit air, and air is oxygen, so leaks always push an oxygen sensor's reading toward lean. That single fact removes the biggest cause category from consideration and leaves three: the sensor is wrong, the wiring is wrong, or the exhaust really does contain unconverted fuel by the time it reaches the rear converter.
One wiring fault deserves naming because it is specific to this sensor type and is missed constantly. A conventional narrow-band zirconia sensor generates its voltage by comparing exhaust oxygen against a reference — and on most designs that reference is ambient air, drawn to the element through the tiny gaps between the strands inside its own wiring harness. Crush that harness under a heat shield, fill the connector with water and let it freeze, or seal the loom with the wrong kind of tape during a repair, and the reference air path closes. The sensor then compares exhaust against exhaust, oxygen looks scarce on both sides, and the output climbs and stays there. It reads rich because it cannot breathe, not because the engine is rich.
Fuel trim data settles the remaining question in seconds. If the engine were genuinely running rich enough to saturate two catalysts in series, long-term fuel trims would be pulling fuel out hard and the upstream sensors would show it. Trims near zero with only the rearmost sensor claiming rich means the engine is fine and the fault is local to that sensor's circuit.
Common causes
- Sensor reference-air path blocked by a crushed harness, a flooded connector or the loom being sealed during a previous repair
- Aged sensor element whose output has drifted high and lost its ability to swing
- Corrosion or water in the rear sensor connector raising the signal voltage
- Signal wire shorted to a low-voltage source or to the sensor ground in the harness
- Silicone or additive contamination of the element, which biases output and typically affects the upstream sensors first
- Genuinely unconverted fuel reaching the rear converter because the front converter is exhausted
- A previous repair that spliced the rear sensor wiring to the wrong circuit
Symptoms
- Check engine light on with normal driveability
- Bank 1 Sensor 3 live voltage parked high and flat with no response to a throttle snap
- Catalyst readiness monitor incomplete, so the vehicle cannot pass an inspection
- Long-term fuel trims sitting near zero, which contradicts the rich claim
- Occasionally a sulphur or rotten-egg smell if the rear converter really is overloaded
- No measurable fuel economy change in most cases
Diagnostic steps
- 1.Read long-term and short-term fuel trims on Bank 1 first. Trims near zero mean the engine is not rich and the fault is in the sensor or its wiring.
- 2.Graph all three Bank 1 sensors together. Sensor 1 should swing, Sensor 2 should be buffered, Sensor 3 should be calmer still — a Sensor 3 pinned high while the others behave is a local fault.
- 3.Snap the throttle and watch Sensor 3. A sensor that does not move at all under a deliberate mixture change is stuck, not merely biased.
- 4.Trace the rear sensor harness looking for a crushed or pinched section under a heat shield or against the floorpan. Any point where the loom has been flattened can close the reference-air path.
- 5.Unplug the connector and inspect it for water, corrosion and any sign it has been repaired with sealing tape or heat-shrink that would block the vent.
- 6.Check the signal line for a short to the sensor ground or to another low-voltage circuit by reading it with the sensor disconnected.
- 7.Inspect the upstream sensors' behaviour for contamination symptoms. If Sensor 1 and Sensor 2 look healthy, a contamination story does not fit and wiring is the better lead.
- 8.Replace the Bank 1 rear sensor once the harness is proven sound, and route the new harness clear of heat shields.
Repair cost
$140 – $450
Rear oxygen sensor replacement generally runs $140 to $450 fitted, with the sensor at $50 to $180. A harness or connector repair on its own is usually $90 to $250. If the rear converter genuinely has fuel reaching it, converter replacement is a much larger job — $500 to $2,000 or more — so prove the sensor and wiring first.
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.