P0040 Code- O2 Sensor Signals Swapped Bank 1 Sensor 1/Bank 2 Sensor 1

What Does Code P0040 Mean

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.

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Possible symptoms

• Engine Light ON (or Service Engine Soon Warning Light)

Possible causes

• HO2S Bank 1 and 2 Sensor 1 harness is open or shorted
• HO2S Bank 1 and 2 Sensor 1 circuit connectors (wrong connection)
• Faulty Engine Control Module (ECM)

Troubleshooting P0040

• Visually inspect the O2 sensors and their connectors

First, make sure the O2 sensor connectors aren’t swapped. This can happen when work has been previously performed. Also check for unplugged or damaged connectors, as well as broken wiring.

• Visually inspect the O2 connections at the PCM

If everything looks OK at the O2 sensors, you’ll want to inspector the O2 sensor connections at the PCM. Look for damaged or crossed wires as well as swapped or broken connectors.

An example of an O2 sensor wiring diagram
(Courtesy: AllData)

• Check O2 sensor performance

If everything checks out OK visually, you’ll want to monitor O2 sensor performance next. This can be done on a scan tool or with a digital multimeter. The best method, however, is with an oscilloscope. This is because scan tool readings are only interpreted values.

If you’re going to use a scan tool, it’s best to view the O2 sensor voltage as a graph. If the upstream O2 sensor and corresponding circuit are operating properly, you will see an oscillating waveform at idle. It should fluctuate between its minimum value of around .1 volts to its maximum value of around .9 volts. An O2 sensor stuck at a fixed bias voltage is a sign of an open O2 sensor circuit or dedicated ground.

The downstream O2 sensor is a different story. If it and the catalytic converter are both operating properly, it should fluctuate very little compared to the upstream sensor. So, don’t be fooled if you don’t see the downstream sensor fluctuating.

An example of upstream and downstream O2 sensor signals
(Courtesy: easterncatalytic.com)

• Check the O2 sensor circuit

If your test results from the section above indicate a problem with the sensor circuit, now is the time to test it.

• Test the signal side of the circuit

Using a digital multimeter set to the ohms, connect one meter lead to the O2 sensor signal pin and the other to the sensor input pin on the PCM. This tests for continuity between the sensor and PCM. If your meter reads OL, there is an open somewhere in the circuit that must be repaired.

You can go a step further by checking voltage drop across the circuit. To do this, backprobe the sensor signal wire at the computer and at the O2 sensor connector. Next, set your digital multimeter set to volts. Allow the vehicle engine to idle and read the voltage drop on the multimeter screen. The voltage drop should not exceed the manufacturer’s specifications. A reading out of specification indicates high resistance in the circuit. The factory wiring diagram will need to be traced to pinpoint the problem.

An example of an typical 4-wire O2 sensor connector diagram
(Courtesy: AllData)

• Test the ground side of the circuit

Check the sensor ground with a digital multimeter set to ohms. Connect one lead to the ground wire pin at the sensor, and the other to the sensor ground pin at the PCM. This tests for continuity between the sensor and PCM. If your meter reads OL, there is an open somewhere in the circuit that must be repaired.

You can also check voltage drop across the ground side of the circuit. To do this, backprobe the ground signal wire at the computer and at the O2 sensor connector. Next, set your digital multimeter set to volts. Allow the vehicle engine to idle and read the voltage drop on the multimeter screen. The voltage drop should not exceed the manufacturer’s specifications. A reading out of specification indicates high resistance in the circuit. The factory wiring diagram will need to be traced to pinpoint the problem.

Note: Some sensors are grounded through the case of the sensor and the exhaust pipe. If you are testing this kinds of sensor, you’ll want to touch the meter lead to the O2 sensor case since there will be no dedicated ground wire.

• Check the PCM

scan tool. Monitor the O2 sensor signal on the scope while simultaneously watching it on the scan tool. If the values don’t match, suspect a problem with the PCM. Keep in mind however, that PCMs rarely go bad. Before condemning the PCM it’s a good idea to double check your work.

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