P0117 and P0306 on my 1996 gmc safari van

Question

changed the thermostat and the van still over heating

Answer 1

P0117 Engine coolant temperature sensor circuit low input

P0306 Misfire number 6 cylinder

When the engine is cold the resistor of coolant temperature sensor is a high resistance,when the engine warms up the resistance is low. when its ,really hot like overheating the resistance is very low and you get  P0117, this can be mechancial condition.where the headgasket has failed, you may have combustion in the radiator Here is a 3.4 where number 6 cylinderhad a misfire and a overheating condition and compression was detected in the radiator