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When O2 Sensor Codes Don't Indicate Problems With the O2 Sensors

O2 sensor

Recently in our forums, user bubblvicious had a problem with a 1997 Ford Thunderbird with a 4.6L V8. The electronic control module (ECM) was throwing error codes for all four oxygen sensors (banks 1 and 2 sensors 1 and 2). As many DIY mechanics would, bubblvicious assumed that this meant all four O2 sensors had gone bad. This is very unlikely and is almost assuredly not the problem with this Thunderbird. On any vehicle, if all O2 sensor codes are throw, it's more likely that the problem is not the sensors themselves, but whatever is causing them to read high or low levels of oxygen in the exhaust.

The codes themselves will often provide the first clue as to what the actual problem may be. They are likely reading "lean" or "rich," depending on the situation.

Lean O2 Code Reasons

If the codes are "lean" (low oxygen), the problem is likely a:

  • vacuum leak,
  • fuel filter problem,
  • a bad mass air flow (MAF) sensor,
  • a leaking exhaust before the sensor, or;
  • an engine misfire.

Rich O2 Code Reasons

If the codes are "rich" (high oxygen), the problem is likely a:

  • leaking fuel injector
  • wiring short to injectors,
  • faulty EVAP system,
  • fuel pump issue,
  • faulty MAF sensor,
  • aftermarket components; or,
  • performance chip conflict.

So if every one of your vehicle's O2 sensors is throwing a reading, it's very likely that something other than a bad sensor is the actual problem. In the case of individual sensor codes, it's more likely that a faulty O2 sensor or it's connection are to blame.