1990 Accord LX, 4 dr, AT, 259,807 miles. Check Engine Light won't come on for 2-3 seconds after turn on ignition key. When it is a very hot day with all windows up (inside car at least 120 degree Fahrenheit or above), CEL will come on for 2-3 seconds right after turn on ignition key. When it is not so hot day and even it is around 100 degree Fahrenheit inside, CEL won't come on when turn key to on. Other than CEL problem, all others are normal, easy to start engine and radio is normal. It has been like this for many years and O2 sensor has been changed with no help to resolve CEL problem. This will fail smog check and my smog check is coming in a couple of months. It was working intermittent two years ago when I had smog check, but not any more. Does anyone know where to check and how to fix it?

 

It may be a loose connection on the pin from the ECU(harness) or cold solder issue on the board/lamp holder itself.
Pull the cluster out and look for damage/corrosion/slop in the connections to the cluster.

 

I took off instrument cluster, inspect circuit board, light bulb, and they all look good. I switched CEL and battery light bulb. Then, I turn key on to check lights. CEL, battery, and other light come on and work as normal. However, when I put back instrument cluster back to its position and turn key on, CEL doesn't come on, but battery light on and off if I wiggled wire. I now know it is connection problem. I inspected wire and don't see anything obviously cause this problem. Put everything back and all lights work like normal now.