Fault code P2278 – O2 sensor signals swapped (Bank 1, Sensor 3/Bank 2, Sensor 3)

Fault code P2278 is called “O2 Sensor Signals Swapped (Bank 1, Sensor 3/Bank 2, Sensor 3)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P2278

Error code P2278 means that the Powertrain Control Module (PCM) has detected that the wiring on the oxygen sensors O₂ has been swapped. The vehicle’s computer uses the oxygen sensor readings to adjust the amount of fuel that needs to be injected into the engine for most efficient operation.

Fault code P2278 – O2 sensor signals swapped (Bank 1, Sensor 3/Bank 2, Sensor 3)

During normal operation, the O₂ sensor responds to changes in the oxygen content of the exhaust stream. Depending on the type of sensor, it detects changes in either signal voltage or electrical resistance. Which the PCM uses to keep the air/fuel ratio as close to the ideal ratio of 14.7:1 as possible.

The PCM monitors the O₂ sensor readings for proper engine operation. For example, if it gives a command to add more fuel to bank 2, but then sees that the oxygen sensor on bank 1 is responding. It processes the information and correlates what it considers to be incorrect data, then this code is triggered.

The P2278 fault code is infrequent and only applies to vehicles with engines that have more than one row of cylinders. If you just have an inline engine, you will not get this code.

The error applies to oxygen sensors O₂ #3. The connection of which is mixed up between each other, more precisely between Bank 1 and Bank 2.

Symptoms of vehicle malfunctions

The main signal that an error P2278 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The “Check Engine” control lamp on the control panel will light up (the code will be recorded in the memory as a fault).
  2. Floating revolutions, as well as attempts to stall at idle.
  3. Failure or emergency mode of operation.
  4. Decrease in engine power output.
  5. Increased fuel consumption.

The degree of severity of fault code P2278 is high. If this error is ignored, there is a high probability of catalytic converter failure. Which in the long run can cost a lot of money.

Factors that can cause this error code

The error code P2278 can mean that one or more of the following problems have occurred:

  • Oxygen sensor connectors #3 (Bank 1 and Bank 2) are mixed up.
  • Oxygen sensor connector wires #3 (Bank 1 and Bank 2) are mixed up.
  • Sometimes the cause is a faulty PCM module.

Almost always the problem starts after repair or replacement of oxygen sensors O₂. Most likely the electrical wires were mixed up during connection.

How to fix or reset OBD-2 code P2278

Some suggested steps for troubleshooting and fix the error code P2278:

  1. Connect the OBD-II scan tool to the vehicle’s diagnostic connector.
  2. Read all stored error codes to find out when and under what circumstances the error appeared.
  3. Clear the errors from the computer memory and test-drive the vehicle to see if code P2278 appears again.
  4. If the error code appears, visually inspect the oxygen sensor connectors and wires #3 (Bank 1 and Bank 2) to see if they are mixed up.
  5. If no problem is found, test the PCM module, and replace it if necessary.

Diagnose and repair of problems

The first thing to do when diagnosing P2278 malfunction is to remember if any work has been done recently on the exhaust and oxygen sensors O₂. If yes, that is most likely the cause of this problem. Most likely, the electrical connectors or wires of the oxygen sensor O₂ No. 3 from Bank 1 to Bank 2 were swapped during connection.

Visually inspect all connectors and wiring to the oxygen sensor O₂. Look to see if the wires are damaged, scuffed or melted, and repair if necessary.

Most likely, the connectors are swapped. You can try swapping these two oxygen wiring connectors as a first step of repair. Then clear the trouble codes and do a drive test to see if the code comes back. If it doesn’t come back, that was probably the problem.

If the P2278 error appears again, carefully inspect the O₂ oxygen sensor wiring and connectors on the PCM side. Make sure the wires are in the correct pin locations going into the PCM. Inspect for misaligned or damaged wires, and repair if necessary.

If necessary, check the continuity of each individual wire from the PCM to the O₂ sensor, repair or replace if necessary.

Failure of the PCM module should be considered as a last resort if all other checks have failed. But if the PCM does fail, take it to a qualified technician for repair or replacement.

On which vehicles does this problem occur most frequently

Fault code P2278 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Ford (Escape)
  • Opel
  • Peugeot (308)
  • Volkswagen

Fault code P2278 can sometimes be found with other errors. The most common are the following: P0040, P0041, P2416, P2417.

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