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One of the less common diagnostic trouble codes you may come across is the P0040 code. Here is some basic information about what this code means for your vehicle.

What Does the P0040 Code Mean?

Diagnostic Trouble Code (DTC) P0040 stands for “O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 2 Sensor 1.”

The P0040 code is set when the powertrain control module or PCM perceives that the wiring harnesses from the sensors have been crossed. It determines this through a logic-based strategy.

On most vehicle platforms it’s not possible to transpose the O2 sensor connections because they simply won’t reach – this transposing of sensor connections typically only happens when the upstream sensor connectors are near enough together to be transposed. Both sensor connections look exactly alike, and if they are near enough to one another and weren’t marked when they were disconnected, they can be transposed.

oxygen sensor on modern car
The P0040 code is set when the powertrain control module or PCM perceives that the wiring harnesses from the sensors have been crossed.

Of course, wiring harness damage or improper wiring harness patching after damage can cause a situation like this if, for example, the wires were connected wrong during a repair, but wire color coding will generally prevent that from happening. Another possibility would be transposing the wires at the ECM/PCM connector, which will only happen if the wires at the connector are extracted from the connector shell and then reinserted.

Since the ECM/PCM is responding to O2 sensor signals to maintain proper mixture on each bank, it’s fairly easy for the ECM/PCM to determine that the sensor connections are reversed. The O2 signals should both be switching very near zero.

Code P0040 refers to a condition wherein the first oxygen (O2) sensors downstream from the engine have swapped wiring. For this trouble code, sensor 1 is the first oxygen sensor in the exhaust manifold or downstream from it. It will be upstream from the catalytic converter. Bank 1 refers to the bank of the engine that contains cylinder #1.

The PCM detects this error by monitoring oxygen sensor readings. For example, if more fuel is injected into bank 2 of the engine but readings show that the bank 1 oxygen sensor is reacting instead of the bank 2 oxygen sensor, the P0040 code is set.

This trouble code is uncommon and only applies to vehicles with engines that have two or more banks of cylinders. Vehicles with inline engines will not get this code. The code P0041 is related to this code.

automotive ecm monitoring o2 readings
The PCM detects this error by monitoring oxygen sensor readings.

Note: The definition of code P0040 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P0040 Code?

  • Misrouted oxygen sensor wiring 
  • Crossed, damaged, and/or shorted oxygen sensor wiring
  • Failed PCM (possible but less likely)

What are the Common Symptoms of the P0040 Code?

How to Diagnose the P0040 Code

Pinpointing the underlying cause of the P0040 code can be tricky, especially without the proper tools or expertise of a mechanic. Most of the time, your best option is to bring your vehicle to an auto repair shop to get an accurate diagnosis from a professional. You may also get more information about possible diagnostic procedures by consulting a repair manual or an online database for factory repair information.

How to Fix the P0040 Code

The P0040 code is a generic trouble code that may apply to various makes and models. It may be possible for vehicles from different manufacturers to present similar symptoms for the same code. However, that does not mean that their repair process will be the same. For instance, the steps to test and repair the underlying cause of a P0040 Chevy code may be completely different from that of a P0040 Ford code’s troubleshooting process.

If you’re an advanced DIYer who’d like to fix this code on your own, it will be helpful to refer to your vehicle’s repair manual or get factory vehicle repair information from an online database.

Fuel Trim and P0040

Fuel trim is the ECM/PCM’s fuel mixture adjustment indicator that is visible on a scan tool live data feed. The ECM/PCM performs fuel trim Correction by modifying injector pulse. For a detailed, technical understanding of fuel trim, read our discussion about long term fuel trim and short term fuel trim.

Long term fuel trim is a coarse adjustment factor that will slowly ratchet in the same direction as short fuel trim until short fuel trim returns to the zero range.

O2 sensor voltage input determines what the ECM/PCM does with fuel trim, and that’s important where the P0040 code is concerned.

If the ECM/PCM is correcting for a rich condition on one bank and that bank keeps getting richer and if it’s correcting for a lean condition on the opposite bank and that bank keeps getting leaner, the fuel trims will be at something like +30 on one bank and -30 on the other bank even though the O2 sensors themselves are providing accurate information.

Products Mentioned in this Guide

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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