Check gauges light
 

I have a 1992 1/2 ton 4x4 Sierra. Recently the check gauges light would occasionally come on and when it did the temp gauge would go up to the red and the oil gauge would stick at 20 psi. Now the check gauges light comes on at start up and the gauges are stuck the previously mentioned levels. I have replaced the coolant sensor in the intake manifold but that did nothing. I put a heat gun on the engine and it is not overheating. Anyone have any ideas?