Skip the auto shop - Our mechanics make house calls

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

check engine light logo

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

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.


Post a question and get free advice from our certified mechanics.


More related articles

P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
How to Buy Fuel Treatments
Adding a fuel additive to your gas tank when you fill up is one way to clean vital engine parts of deposits, improve the performance of your engine,...
P2422 OBD-II Trouble Code: Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed
Diagnostic Trouble Code (DTC): P2422 P2422 code definition Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed Related Trouble Codes: P2441: EVAP Vent Valve Stuck Open EVAP trouble...

Related questions

Q: Check engine light on due to gas cap.

You should have your fuel vapor control system for leaks by a mechanic that can fill the system with smoke from a smoke machine that helps to locate the leak in the system. The purge control valve is the most...

Q: Car won't accelerate past 3000rpm

This issue could be caused by a variety of things. I would suggest by checking to be sure that the idle air control module is working properly. Also, check to be sure that the throttle body is delivering...

Q: What is the best fix for a catalyst system efficiency below threshold (bank 2)

Trouble code P0430 is for a catalyst on the drivers side. The sensor after the converter may be bad and not responsive enough or the catalyst itself may be bad. The catalyst should be replaced if the converter rattles, there...