My 93 Altima has 2 o2 sensors. I noticed that the one before the cat converter was unplugged. Would this cause any problems? There was no check engine light.
Also, now that I have plugged it back in, should I notice any performance or fuel economy change?
The one before the Cat is the upstream O2 sensor. It measured the percent of misfire (or unburned fuel) going to the cat.. The one after the Cat is the downstream O2 sensor. It measures the how efficiently the Cat is working.
I believe it was after 1996 models when OBII standards came in that mandated an error message if the sensor was unplugged.
It is also possible that a previous owner has put a resistor in the harness and unplugged the sensor so the computer would read a more desirable range (due to the resistor), then allow for a richer mixture.
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