A stuck thermostat is unlikely. Check serpentine belt condition and tension, then diagnose the cooling fan circuit: fuses in both fuse boxes, relay, coolant temperature sensor and connector, wiring, and fan motor. Confirm the fan runs with the A/C on or by applying 12 volts directly. Fix the overheating first; if the fourth-gear issue remains, have the trouble codes read.
A 1998 Saturn SC2 overheats in stop-and-go traffic, the engine light comes on, and fourth gear becomes unavailable until the car cools. Coolant was recently flushed and levels look normal; transmission fluid was replaced at 65,000 miles. The expert said a thermostat would not cause only intermittent overheating and pointed to belt drive of the water pump and, more likely, the cooling fan system, including fuses, relay, coolant temperature sensor, wiring, and fan motor. Cooling should be corrected first, then codes pulled if shifting still fails.
