1990 S10 4.3 temperature gauge reads High replaced both sensors and thermostat.

This could be a few things but you need to verify if the engine is actually over heating or if it's a gauge issue. You can order a cheaper laser temperature gun online. Point it to where the coolant temp sensor is and see if it lines up with what the gauge is reading. If it is indeed over heating then you need to start with basics like making sure the fan is workings, the radiator isn't clogged and the water pump is pumping. Alex

Was the temperature gauge *sending unit* on the drivers side cylinder head replaced? Is that one of the sensors you're referring to? Defective 'new' thermostats are not uncommon.