I have a bad rear O2 sensor so my CEL is always on because of that. Today however, the CEL turned off immediately after starting the car and stayed off for the 50 mile drive I did. When I got where I was going, I shut off the engine and turned the key back on, and the CEL did not turn on, while all other accessory power did. I then turned the key back off, back on, and started the engine. Now it is on solid again. How could this possibly happen?

Related incident a few weeks ago - I had just reset the CEL after replacing an ignition coil, and when I started driving immediately after resetting it, the CEL periodically flashed very randomly. It would stop, steady on, off, random flashing, etc. This was definitely not a steady flashing that you get with a misfire or urgent problem. I just reset the light again and it behaved normally thereafter.


So, my question is, could a problem with the ECU be causing the light to behave unpredictably? And could that problem also have caused my four coil failures? My understanding of the light is that the socket is soldered directly to the instrument board and so a loose wire or something simple seems unlikely?

 

The CEL can turn itse;f on and off sometimes.

One test to make sure it works properly is to turn the car, but don't start it. When they key is on, but the engine is off (KOEO), the CEL should ALWAYS be on.

You can also hook up a scan tool. It will tell you whether the CEL is supposed to be on or off. For example, if the scanner says the CEL is supposed to be on, but the CEL isn't actually on, then there may be a problem. Note that it CAN command the CEL OFF even if there are codes stored. For example, a bad knock sensor code P0325 will NOT turn on the CEL, but it will still throw a code.

 

That's bad then because it did not light the first time I turned the key on when I got back. But did the second time. What does that mean?

 

You should buy a scan tool.

I bought mone from Amazon for 22 bucks.

BAFX brand. I plugs in to your obd 2 port. It's blue tooth. Uses your phone as the display. Either Android or Apple models.

 

Maybe the light bulb for the cel is bad, swap the cel bulb with one of the lamps next to it and see what happens.