2004 Toyota Corolla Check Engine Light

I bought a 2004 Toyota Corolla new. It is a great car but the Check Engine light on the dash is driving me crazy.

In Nov 2007, at 88,000 miles, the light went on for the first time. My mechanic installed a new oxygen sensor. Everyting was fine until Dec 2008, at 112,000 miles, when the light went on again. Another oxygen sensor was installed. The light remained off for about two weeks, then came on. Since then the light cycles on and off for about three weeks at a time. I’ve replaced the gas cap but the light still cycles off and on. Any suggestions will be appreciated. John

If the light comes on there is a code, or codes, stored in the computer. Unless you have the code(s) read you are just guessing. It may be something totally unrelated to the gas cap or the O2 sensors. Many parts stores will read the codes free, or you can let your mechanic read them.

Thanks for your reply. My mechanic has read the codes and they show a problem with the sensor when the Check Engine light is on. However, if we do nothing, the light will go off in a couple of weeks and the codes don’t show a problem. This cycle just keeps repeating. Any other ideas? Thanks again, John

Make sure that he is replacing the right (or both) o2 sensors. Your car you should have two, one before the CAT and one after.

Thanks. I’ll ask the mechanic if he replaced both O2 sensors. John

The problem your are could also be due to a fault in the circuit to the sensor but I assume your service tech would be aware of that. From your statements though it sounds to me there is an intermittent problem somewhere. If the code clears the CEL light will go out after enough cycles have occured with no trouble seen by the ECU. When the trouble comes back the light will be set on again. It would help us if we knew what specific code you are seeing.

Thanks. I’ll see if my mechanic can give me the codes (should be no problem since the light is now on and has been for a week). I’ll report back in a couple of days due to business committments. John

The rear O2 sensor is hardwired in the vehicle, no connector so it is quite difficult to replace. If you have had a lot of rain in your area lately, it could be due to moisture in the front O2 connector. It may just need cleaning and sealing around the edge with a dielectric grease.

It would help to know the code too.

My mechanic has read the codes and they show a problem with the sensor when the Check Engine light is on

No they show a sensor reporting back a reading that is not expected. It might be the sensor or what it is sensing.

Please get the actual code.  It will be in the format of [P0123].  Many auto part stores will read the code for free.  Get the code and post it back here.

It would be helpful to get the actural code. What you are getting is your mechanics interpretation of the code. The code should be in the format P0123.

You don’t want silicon dielectric grease, or sealant, anywhere near the oxygen sensor. It’s poison.

Not on the sensor, on the connector, which is about a foot away from the sensor.