2000 tahoe temp guage malfunction

The temp guage works about 5% of the time. When the guage does not work properly it will not blow hot air. When the guage does work and vehicle is at running temp it blows hot air just fine.
Additional info: the AC does not work either. Was told by mechanic that problem is under the dash with a leak. Forgot term he used.
Additional info: Not all warning lights on instrument panel work either.
Additional info: Service engine soon light is on.
Any help on which part to replace would be great to fix the guage or instrument lights.