Temperature gauge fluctuating, check engine light staying on on 1993 Ford Explorer

Have just had cylinder heads done, a rebuilt transmission, new thermostat, new radiator and clutch fan put in. When driving, the temperature gauge goes way up and down, last night check engine light came on and is staying on. Garage hooked up an auto scanner and the code was "cylinder balance", the mechanic did not know what this meant and could not tell me why the temperature gauge was fluctuating. He said it was nothing to worry about, but I am.

Asked by for the 1993 Ford Explorer
You have not been given the right answer. If your check engine light is on, there can certainly be some serious problems, and after such a major service, the car should be running perfectly. The work done has not been completed until your car is running well and the check engine light is off.
2 more answers , 1 more comment
could be a bad ground off to the thermost or on engine when they done the head work hope this helps
I think you mean the temp sending unit may have a bad connection, etc. It's fairly connon for the check engine light to come on after major repairs. mechanics in their hast to finish the job neglect to reconnect wires/ cross wires/disconnect grounds/etc. A simple obd2 scan should tell the tale, as long as you can trust whoever youget to scan it.
fluctuating also can be air in the cooling system not giving proper flow and could also be thermostat cooling system need to bleed air out or off timing