P0040: O2 Sensor Signals Swapped — Bank 1 Sensor 1 / Bank 2 Sensor 1
What to do
Mention any recent exhaust or sensor work when booking this in — that history usually solves it immediately.
What this code means
The module has concluded that the two upstream oxygen sensors are reporting each other's readings. Nine times out of ten this is exactly what it sounds like: the connectors were crossed during a previous repair. It is one of the few codes that points at workmanship rather than at a failed part, so establish what was worked on recently before testing anything.
Likely causes
Start here
Less common
- Wiring harness repaired incorrectly · moderate
- Faulty oxygen sensor · easy
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
Be the first to write this up
Nothing posted about this yet. Whatever you found is worth writing down — even a dead end saves someone the same afternoon.
What makes one worth reading:
- The car — make, model, year and engine. The same fault behaves differently on each.
- What you measured, not only what you replaced.
- What actually fixed it — or that nothing did, which is worth knowing too.
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