96chevy cavalier ohv 2.2L that cools right down to 40 degrees c at highway speeds plus no heat lower city you have and it run normal operating temp thermostat changed not stuck open?

- Check the coolant temperature sensor. The coolant temperature sensor sends a signal to the engine control unit (ECU), which uses it to control the engine's cooling system. If the sensor is faulty, it may be sending an incorrect signal to the ECU, causing the engine to run cooler than it should.

- Check the thermostat. The thermostat is a valve that regulates the flow of coolant through the engine. If the thermostat is stuck open, it will allow coolant to flow through the engine too quickly, causing it to cool down too much.

- Check the water pump. The water pump circulates coolant through the engine. If the water pump is faulty, it may not be circulating coolant properly, causing the engine to overheat.

- Check the radiator. The radiator is a heat exchanger that helps to cool the coolant. If the radiator is clogged or damaged, it may not be able to dissipate heat properly, causing the engine to overheat.

- Check the heater core. The heater core is a heat exchanger that transfers heat from the engine to the passenger compartment. If the heater core is clogged or damaged, it may not be able to transfer heat properly, causing the passenger compartment to be cold.