94 Silverado- Check Gauges light stays on

The "Check Gauges" light on the intrument panel has been on ever since my dad bought his used 94 Silverado 2 years ago.

All the gauges seem to work fine. The only odd thing I notice is that the tachometer seems to show slightly lower RPMs than the engine is actually running at.

Any ideas what could cause this "Check Gauges" light to stay on?

Thanks,
MAP74