1994 Chevy Cavalier Check Engine light on, rough idling

Hello! As of about a month ago, my check engine (CE) light would intermittently come on and off, without the car feeling any different. About last month till now the CE light comes on after about a minute of driving, and when dark, the headlight dim a little right before the CE light comes on.

I tried jumpering the two pins to make the computer flash the error code, however it doesn't even flash anything! Not even the standard 21 if there is nothing wrong.

The car has also started to really idle rough in drive and reverse since last month also. (I'm talking full out interior squeaking shakes that can only be stopped by pressing on the dash/center console!)

I have also noted in the last month that the cars temperature gauge does not move as it used to.
At normal operating temperature, it is usually 1/4 the way up, and when stopped and idling, would climb up to 1/2, and then the fan would kick on, and go back down to the 1/4 range. But as of lately, it will climb to normal temp. (1/4) and stay there even if I was sitting in stop and go traffic on a 90F day!
(It didn't do this staying at 1/4 temp thing before the CE light was on)

My ideas so far was that if the car has a sensor to turn on the fan, that it might be stuck in the on position, but I don't have any real clue.

Thank you!

Thomas