if coolant sensor is going bad-will check engine light come on

Possibly, scan for the trouble code to be certain.

Possibly. Or faulty wiring going to the sensor. Or faulty wiring somewhere else interfering with the signal from the coolant sensor. Or a bad ground somewhere. I'm going to assume you've made sure the coolant level is where it should be. Scanning is a good start, but don't start replacing parts until you know they're actually faulty. On older cars these sort of intermittent problems often lie elsewhere in the wiring harness.

Thanks Mark-most helpful answer.