I have an 05 Cavalier. The fuel gauge was intermittent and finally failed. I was told by a mechanic to find the connector for the gauge and unplug it. If the gauge moves to the full position, the sender is bad. If the gauge stays low, its the sender. To make matters worse, now my speed and temp gauges malfunction every once in awhile. They will read incorrectly while driving and when the motor is off. They usually start working after the car is turned off and restarted. I am at a loss on how to proceed. I don't want to replace the cluster unless I can be sure it'* the issue. I read that the signals to the gauges are not electrical, they are data. Is that correct? Could I have a ground or communications issue? Any help would be appreciated.

your speed is likely from the pcm so if your trans is shifting properly when that happens the vss is sending the proper signal to the pcm. the test you were going to do for the fuel is a good one, most cars dont send the fuel level or oil pressure to the pcm. the temp does go to the pcm though so to me sounds like a cluster issue. you could try to inspect the connector and wires on the cluster before you buy a new one, reseat it a couple times see if that changes it