1999 Mazda Protege LX 1.8L P0140

This is a cool site and very helpful for the home mechanic.

I have been having CEL problems with my 1999 Mazda Protege LX touring 1.8L. The light comes on and usually does not turn off unless I unhook the battery or use my code reader to turn it off. It passed an e-test with flying colors after a turned off the CEL before bringing it in.

Anyways basically the light comes on after going on to the highway and specifically after filling up the gas tank. I have tried different gas stations, but without fail, the CEL will be off before filling up and as soon as I fill up it will come on within 20 minutes of driving.

So I picked up and Elm327 with the laptop interface do read the code. It throws a P0140 which from my understanding means the post CAT O2 sensor is sending any information. My question is why would this correlate so much with filling up? In fact I have even filled up and let the CEL flip on and then turned it off right after, for the rest of the tank it will never come on until I fill up again.

I'm guessing it is the O2 sensor or the wiring between, but I just don't understand the correlation with filling up.

If anyone has any ideas or has seen this before please help me out, I don't want to start buying sensors and what not only to find out that isn't the real problem.

Thanks

It sounds to me as if you have a dead O2 sensor or a wiring problem. Does your new diagnostic program have a live data stream? If it does, there will be a line of data for O2 sensor bank one sensor 2. You can watch the voltage to see if it is doing anything. I dn't know what it will look like on your Mazda, but on a Toyota a dead O2 sensor will prompt the computer to show a substitute value that never changes. The value should change when you rev the engine. If it doesn't, you likely have a dead sensor.