P0040 OBD-II Trouble Code: O2 Sensor Signals Swapped Sensor 1

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P0040 code definition

P0040 indicates that there is an issue with the oxygen sensor sensor 1 circuit. This code can be in conjunction with P0041

What the P0040 code means

P0040 is a specific code indicating a problem with the bank 1 sensors. The engine control unit (ECU) has detected swapped sensor signals for sensor 1 between bank 1 and bank 2. This may be indicated by a flashing Check Engine Light and the vehicle’s ECU triggering a failsafe mode.

What causes the P0040 code?

Oxygen sensor bank 1 signals switched can have several causes:

  • The connectors for bank 1 sensor 1 and bank 2 sensor 1 have been swapped
  • Crossed oxygen sensor wiring
  • ECU failure
  • Open or short in oxygen sensor wiring for bank 1 sensors

The most common cause is the connectors being swapped during installation or removal.

What are the symptoms of the P0040 code?

The Check Engine Light generally will proceed symptoms of P0040. The vehicle may begin to run roughly, feel sluggish, run low on power, or have poor fuel economy. P0040 may cause a flashing Check Engine Light, meaning potential damage to the catalytic converter. After this code, the car’s ECU may go into failsafe mode to prevent further damage to the vehicle.

How does a mechanic diagnose the P0040 code?

  • The mechanic will begin by hooking up a scan tool into the car's OBD-II port and confirming P0040 is present.

  • If P0040 is confirmed, the freeze frame data will be observed to tell us what conditions the code was set under.

  • The code will be cleared and the vehicle taken on a road test by the mechanic to duplicate the code.

  • After the code is observed, the mechanic will perform a visual inspection and confirm the connectors are going to the correct sensors.

  • If the connectors are in the proper location, the wiring will be both visually inspected for shorts and a continuity test will be performed to ensure the wiring is not open or shorted together.

  • If all else fails the mechanic will perform the manufacturer's specific test for the vehicle’s ECU.

Common mistakes when diagnosing the P0040 code

P0040 is an uncommon code that is usually seen from simply plugging the sensor connectors in the other side’s sensor. This code should be diagnosed by the books and care should be taken to make sure the connectors are plugged into the right sides before a more extensive diagnostic is performed.

How serious is the P0040 code?

ECU failure and catalytic converter damage are possible, so this code should be diagnosed promptly. The vehicle’s ECU may put the vehicle into a failsafe mode. With all of these symptoms, normal operation of the vehicle will be impossible and not recommended until the vehicle is repaired.

What repairs can fix the P0040 code?

Generally, this repair is simply a swapped connector but there are several potential repairs:

  • Installing connectors on proper sensors
  • Repairing or replacing wiring
  • Repairing or replacing ECU

Additional comments for consideration regarding the P0040 code

In my experience, P0040 is not a code we see often. It is generally due to replacement of oxygen sensors and the connectors simply getting swapped to the wrong side.

Need help with a P0040 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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