OBD-II trouble code
P2277: O2 Sensor Signal Biased/Stuck Rich (Bank 2, Sensor 3)
The rearmost Bank 2 oxygen sensor is pinned high. Because the matching Bank 1 sensor is normally the identical part, swapping the two is a free test that proves whether the sensor or the car is at fault.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $480
- DIY difficulty
- Intermediate DIY
Browse every code in P2200–P229F, or start from the full code library.
What does P2277 mean?
P2277 is the Bank 2 counterpart of the rear stuck-rich fault: the third sensor on the bank without cylinder 1 is reporting an oxygen-poor stream and will not move off that reading. Electrically the circuit is intact, so this is a plausibility judgement rather than a short or an open.
The test worth knowing about here exists only because the vehicle has two banks. On the overwhelming majority of V engines, the Bank 1 and Bank 2 rear sensors are the same part number with the same connector and often the same harness length. Unbolt both, swap them side to side, clear the codes and drive a cycle. If the rich bias follows the sensor to the other bank, the sensor is faulty and the answer cost nothing but time. If the code stays on Bank 2 with a different sensor fitted, the sensor is innocent and the fault is in the Bank 2 wiring or in the exhaust itself. Very few oxygen sensor codes offer a definitive parts-swap test using parts already on the vehicle, and this one does.
The second thing that sets this address apart is where its connector lives. On many vehicles the Bank 2 rear sensor plug is clipped near the rear of the transmission or beside a heat shield above the axle — right in the working area for transmission, driveshaft and exhaust jobs. It is routinely unplugged for access and occasionally not plugged back in, or plugged back in without being fully latched. If this code appeared within a few hundred miles of underbody work, check the connector before anything else, because the repair is free.
In terms of consequence this is the mildest code in the group. It does not trim fuel, it does not affect cold-start enrichment, and it does not put anything at mechanical risk. What it blocks is the Bank 2 catalyst monitor, which is enough to stop an inspection but nothing more.
Common causes
- Connector left unplugged or not fully latched after transmission, driveshaft or exhaust work
- Aged Bank 2 rear sensor whose output has drifted high and lost its swing
- Water or corrosion in the Bank 2 rear connector raising the signal voltage
- Signal wire shorted to sensor ground or another low-voltage circuit along the tunnel routing
- Blocked sensor reference-air path from a crushed section of the Bank 2 harness
- Genuinely unconverted fuel reaching the Bank 2 rear converter because the front one is spent
- A previous wiring repair on the rear sensor loom terminated to the wrong circuit
Symptoms
- Check engine light on, vehicle drives normally
- Bank 2 Sensor 3 voltage parked high and unresponsive to a throttle snap
- Bank 1 Sensor 3 behaving normally at the same moment
- Catalyst readiness monitor incomplete for Bank 2
- Failed emissions inspection on readiness
- Bank 2 fuel trims sitting near zero despite the rich claim
Diagnostic steps
- 1.Ask what underbody work the vehicle has had recently, then find the Bank 2 rear sensor connector and confirm it is fully latched and undamaged. This is the cheapest possible outcome and it is common.
- 2.Read Bank 2 fuel trims. Near zero means the engine is not rich and only this sensor thinks otherwise.
- 3.Graph Bank 1 Sensor 3 against Bank 2 Sensor 3. A high flat Bank 2 trace against a normal Bank 1 trace confirms the fault is local.
- 4.Swap the two rear sensors between banks, clear codes and drive a cycle. If the code moves to Bank 1, the sensor is faulty; if it stays on Bank 2, the sensor is not.
- 5.Inspect the Bank 2 rear harness for a crushed section or a repair that sealed the loom, either of which can block the sensor's reference-air path and bias it high.
- 6.With the sensor unplugged, check the signal line for a short to ground or to another low-voltage circuit.
- 7.Verify the Bank 2 upstream sensors are behaving before considering the rear converter, since a genuinely rich stream would show there first.
- 8.Replace the Bank 2 rear sensor only after the swap test or the wiring checks have pointed at it.
Repair cost
$0 – $480
A connector that was never plugged back in costs nothing to fix, which is why the low end is zero. Bank 2 rear sensor replacement typically runs $150 to $480 fitted, with the sensor at $50 to $180. Wiring repair along the tunnel routing is usually $100 to $280. Rear converter replacement, if the rich reading turns out to be real, is far more and should only follow a proven sensor and harness.
Estimate your repair
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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.