Temp gauge is pegged to the top and stuck. check gauge light is on

i'd change the engine temperature sender unit , or just disconnect it for a while and see if the gauge symptoms disappear , if they do then change the temp sender unit,if the symptoms remain i'd say get the OBD codes read as may have a faulty dash component

If the truck were newer (1996+) it would have OBDll functionality ... then if the sensor went out ... odds are you would get a check engine light also ... since the signal not only goes to the gauge but to the computer.
I do not know of any easy tests without removing parts.
However .... there may be a chance that it is neither the gauge or sensor but a stuck open thermostat which prevents the engine from heating up ... maybe even after 5 minutes.
There are some things to check the thermostat under cold engine conditions without having to remove parts:

Helpful?
Good luck on your troubleshooting!

probably a faulty temperature sender unit
temp sensor unit which controls the fan)

You could replace the sending unit to cross it off the list of possible problems.
You could check resistance cold and then with the engine at normal temp too. The gauge on the dash could be faulty too.

Yes the thermostat could be stuck closed or you could have a pressure leak in the coolant system such as the radiator cap and gaskets.

go to dollar store buy liquid themomitor put it in the fill cap check temp. gauge might be bad in car radiator mite be pluged

Hello,

You problem sounds like a "Blend Door" issue, you have these doors that open & close and everything in between to adjust air temp. These doors have a "peg" that is connected to an arm or wire just like on a ten speed bike to shift gears, that peg may have broken off.

Another issue may be a stuck open thermostat, if you have a temp gauge is it running at normal temp? If the thermostat is stuck open then heat may become an issue.

I have found several 2007 Chevrolet Impala blend door issues here in fixya.com.

Hope this helps.

A faulty temperature sending unit. It should be located on or near the thermostat housing. If any of your gauges reads zero, the check gauges light will come on. Sometimes even if the fuel gauge gets too low, it will trigger the light. It may be the connection at the sending unit. It may have become loose so wiggle it and make sure it is securely connected to the sending unit. If the temp. gauge needle pegs at the top of the gauge, it means the sending unit has grounded out and is faulty. If it jumps, reads off or not at all it could be a faulty sender or a bad connection.

aFTER A HEATER CORE WENT OUT THE 454 OVERHEATED. hEATER COR REPLACED TEMP GUAGE STUCK PAST 160. cOOLING WORKS FINE NOW, TEMP GUAGE JUST PEGGED ON HIGH
wHAT NOW.....

yes. but check every thing else first. coolant level being the most important. then the thermostat.