OBD-II trouble code
P2278: O2 Sensor Signals Swapped (Bank 1 Sensor 3 / Bank 2 Sensor 3)
The module has concluded the two rearmost oxygen sensors are connected to each other's circuits. This is not a component failure — it is an assembly error, and the fix is usually free.
Quick facts
- System
- Powertrain
- Category
- Oxygen Sensor
- Severity
- Low severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $0 – $250
- DIY difficulty
- Intermediate DIY
Browse every code in P2200–P229F, or start from the full code library.
What does P2278 mean?
Almost every oxygen sensor code describes something that broke. This one describes something that was built wrong. P2278 sets when the engine control module decides that the signal arriving on the Bank 1 rear sensor circuit is actually coming from the Bank 2 sensor, and vice versa — the two are crossed.
The module can reach that conclusion because it has a way to address the banks separately. Either it applies a brief bank-specific fuelling change and watches which input responds, or it uses a natural asymmetry — a deceleration fuel cut, a bank-specific misfire, a different warm-up rate — and checks that the right sensor reacts. When the response consistently arrives on the wrong circuit, there is no plausible failure mode that produces that result. Crossed wiring is the only explanation, and the module says so plainly.
That makes the repair history the diagnostic. This code virtually never appears on a vehicle that has been left alone. It shows up after catalytic converter replacement, after an exhaust section has been renewed, after both rear sensors were replaced at the same time, or after a wiring repair on the rear loom. The mechanism is mundane: the two sensors are usually the same part number with identical connectors, so nothing physically stops either plug going into either socket, and under a vehicle on a lift the two harnesses look the same.
The important practical instruction is what not to do. Do not replace sensors on this code. Two new sensors fitted the same crossed way will set it again, and the owner will have paid for parts that were never faulty. Find the two connectors, work out which belongs to which bank from the wiring diagram, and swap them back.
Common causes
- Rear sensor connectors crossed during catalytic converter or exhaust section replacement
- Both rear sensors replaced at once and reconnected to the wrong sides
- A wiring repair on the rear sensor loom spliced to the opposite bank's circuit
- Aftermarket or universal sensors fitted with extended leads routed to the wrong connector
- Exhaust components installed on the wrong sides so the harnesses reach the opposite bungs
- A replacement or reflashed control module carrying a calibration for a different exhaust layout
- A harness from a different model variant fitted during a repair
Symptoms
- Check engine light on with no change in driveability
- Code appearing shortly after exhaust, converter or oxygen sensor work
- Readiness monitors that will not complete
- Failed emissions inspection on readiness rather than measured output
- Rear sensor readings that respond to changes on the opposite bank
- Frequently one or more companion catalyst efficiency codes on the wrong bank
Diagnostic steps
- 1.Establish what work the vehicle has had recently. This code follows exhaust, converter, sensor and harness repairs and rarely appears without one.
- 2.Get the wiring diagram and identify which connector and which pins belong to the Bank 1 rear sensor and which to Bank 2. Colour alone is not reliable on a repaired loom.
- 3.Trace each rear sensor's physical lead from its bung back to the connector it is plugged into, and confirm the sensor in the Bank 1 pipe reaches the Bank 1 circuit.
- 4.If the vehicle supports a bank-specific test, run it and watch which input responds; the sensor that reacts to a Bank 1 event should be the Bank 1 sensor.
- 5.Look for evidence of a previous splice or repair in the rear loom, which is the second most common cause after crossed plugs.
- 6.If the wiring is correct and the sensors are in the right pipes, confirm the exhaust parts themselves are on the correct sides — a mirrored assembly produces the same result.
- 7.Correct the crossed connection, clear the code and drive a full cycle so the monitors can re-run.
- 8.Only if the wiring is verified correct and the code returns should the control module's calibration for this vehicle's exhaust layout be questioned.
Repair cost
$0 – $250
Unplugging two connectors and swapping them back costs nothing, and that is the usual repair — hence a low end of zero. If a shop has to trace the loom against a wiring diagram, expect $90 to $200 in diagnostic time. Repairing a splice that was made to the wrong circuit runs roughly $100 to $250. Replacing sensors on this code is money spent on parts that are not faulty.
Estimate your repair
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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.