Temperature Gauge Malfunction Chevy 3800

I removed and replaced the lower intake gasket. Im having a problem with getting my temperature gauge to work. I replaced the sending unit. When I plug in the connector and turn on the key. The gauge goes straight to hot and the hot light comes on, even though the engine is cool. And the check engine light comes on. I disconnect the sending unit and the gauge is not working, the HOT light is not on, but the "check engine" light stays on. None of the wires seem to be touching ground and they seem to be in good shape. What is my problem? How do I get my gauge to work properly?

You might want to get the car scanned to see if it has any trouble codes related to your problem. That can help aid in diagnosis.

Otherwise, you could just have a faulty gauge in the instrument cluster. If it worked normally before you did the other repairs, then it has to be something you did. Maybe it is connected improperly, or you might want to check the wiring one more time.

If you installed an aftermarket temp sensor, that can be the cause as well, as sometimes they are built backwards since they are inferior parts. Get a GM unit to be sure you have a good one.

If it is a gauge problem itself, then you would need to replace the entire instrument cluster.

Is it true that enhancing and polishing your 3800 performance heads will void the manufacturer's warranty?

Any internal engine work that is not from the factory can indeed void any and all warranties. Would a technician be able to tell there was head work done? Probably not, unless there is a really close inspection of the cylinder heads.

If that is all that is done to the engine, I wouldn't worry about it too much on voiding the warranty, unless you have an actual cylinder head problem where they would be taken apart.