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Diagnose/P0040
P0040Medium SeverityEmissions

O2 Sensor Signals Swapped (Bank 1 Sensor 1 / Bank 2 Sensor 1)

What This Means

The upstream oxygen sensor signals for bank 1 and bank 2 appear to be reversed. This usually happens after engine work where the sensor connectors were swapped.

Common Symptoms

  • Poor fuel economy
  • Rough idle
  • Failed emissions test
  • Check engine light on

Common Causes

  • O2 sensor connectors swapped during repair
  • Wiring crossed at splice
  • Incorrect sensor installation
  • Harness routing error
  • Aftermarket wiring mistake

Estimated Repair Cost

$50 – $300
Parts and labor (varies by vehicle and location)
Difficulty:Easy

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For informational purposes only. Diagnostic trouble codes are starting points for diagnosis, not definitive answers. Always consult a certified mechanic for proper diagnosis and repair. Estimated costs vary by vehicle, location, and shop rates.