# AC Thermostat and Evaporator Sensor Replacement in Upland > AC freezes up and turns warm on long drives? Route 66 Auto Repair in Upland tests the evaporator thermostat or temperature sensor and replaces it. Source: https://route66autorepairshop.com/cooling-heating-ac-services/ac-thermostat-replacement Updated: 2026-09-17 Business: Route 66 Auto Repair, 1775 W Foothill Blvd, Upland, CA 91786, (909) 608-7699 Hours: Monday to Friday 8 AM–5 PM, Saturday 8 AM–2 PM, Sunday closed **TLDR:** This part keeps the **evaporator from icing**. When it fails, cooling fades on long drives. We compare its **reading to actual temperature** before replacement. The AC thermostat, also called the evaporator temperature sensor or thermal switch, keeps your evaporator from freezing. Route 66 Auto Repair in Upland replaces it when the AC gets cold, then turns warm after a long drive, or cycles too often. It is not the engine thermostat. We test its reading against real evaporator temperature before replacing it, and the written estimate lists the access work. ## What is an AC thermostat? The evaporator gets cold enough to freeze the moisture it collects. If ice builds up, air cannot pass through, and the vents go warm even though the compressor is running. The AC thermostat measures the coil temperature and tells the system to back off the compressor before that happens. On older cars it is a switch with a capillary tube pushed into the coil fins. On newer cars it is an electronic sensor clipped to the evaporator that reports to the climate control module. ## What are the signs it is failing? - AC starts cold, then weakens after twenty or thirty minutes on the freeway - Airflow drops even though the blower sounds normal, then recovers after the car sits - Water drips from under the dash after a long drive as ice melts - Compressor never engages because the sensor reports a frozen coil - Compressor cycles on and off too quickly ## How do we test it? On electronic sensors, we read the reported evaporator temperature on the scan tool and compare it with a probe at the center vent and the low side pressure. A reading that does not track reality, or one that is stuck, points to the sensor or its wiring. On capillary switches, we check whether the switch opens and closes at the right point. We also rule out a moisture problem at the [expansion valve](https://route66autorepairshop.com/cooling-heating-ac-services/ac-expansion-valve-repair) and a low charge, both of which can freeze the coil too. ## Our replacement process 1. **Confirm icing:** We run the AC under load and watch airflow, vent temperature and sensor data. 2. **Test the sensor:** Its reading and wiring are checked against real measurements. 3. **Gain access:** Depending on the car, a glovebox, trim panel or blower motor is removed. 4. **Replace and road test:** The sensor is installed in the correct position and a long road test confirms steady cooling. ## What's included in AC thermostat replacement - Load test with sensor data logging - Sensor reading and wiring test - Access panel removal and reinstall - AC thermostat or evaporator sensor - Extended road test with vent temperature log ## How pricing and timing work Many sensors are reached by removing the blower motor or a small panel, which keeps the job short. Some sit inside the heater box and need more disassembly. Your estimate reflects which one you have. The refrigerant system usually stays sealed. If you are looking for the engine part that controls coolant flow, see [thermostat replacement](https://route66autorepairshop.com/cooling-heating-ac-services/thermostat-replacement). For related sensor issues, see [AC system temperature sensor repair](https://route66autorepairshop.com/cooling-heating-ac-services/ac-system-temperature-sensor-repair) and [car AC repair](https://route66autorepairshop.com/cooling-heating-ac-services/car-ac-repair). ## Frequently asked questions ### Is the AC thermostat the same as the engine thermostat? No. The engine thermostat controls coolant flow to the radiator. The AC thermostat measures evaporator temperature to prevent icing. They are separate parts with separate symptoms. ### Why does my AC work again after I stop for a while? The ice on the evaporator melts while the car sits, so airflow returns. That pattern strongly suggests an icing problem. ### Can I drive with a frozen evaporator? It will not harm the engine. Turn the AC off for a few minutes with the fan on to melt the ice, and schedule a check. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.