# Radiator Thermostat Replacement in Upland > Engine running cold, hot or slow to warm up? Route 66 Auto Repair in Upland tests the thermostat with scan data and replaces it with the right rating. Source: https://route66autorepairshop.com/cooling-heating-ac-services/radiator-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:** Stuck **open** means slow warm-up and weak heat. Stuck **closed** means fast overheating. We read the warm-up curve before replacing the **thermostat**. The thermostat is the valve between the engine and the radiator. It stays closed while the engine warms up, then opens to send coolant to the radiator. Route 66 Auto Repair in Upland checks how the thermostat behaves using scan data and temperature readings, and replaces it with the correct temperature rating when it sticks open or closed. ## What are the signs of a bad thermostat? - Gauge stays low on the freeway and the heater is lukewarm - Check engine light for a warm-up or coolant temperature code - Engine overheats within minutes while the upper hose stays cool - Gauge swings from normal to hot and back - Fuel economy drops because the engine never fully warms up ## What happens when a thermostat sticks open or closed? A thermostat stuck open lets coolant flow to the radiator from a cold start, so the engine takes too long to warm up. The computer runs it richer, heat is weak, and many cars set a code. It is not an emergency, but it wastes fuel. A thermostat stuck closed traps heat in the engine. The gauge climbs fast and the radiator stays cool. That one is a need-now repair. A [check engine light diagnostic](https://route66autorepairshop.com/engine-services/check-engine-light-diagnostics) often confirms which way it failed. If the thermostat sits in a housing that is also leaking, see [thermostat housing repair](https://route66autorepairshop.com/cooling-heating-ac-services/thermostat-housing-repair). Thermostats carry a temperature rating, usually stamped on the flange. Most passenger vehicles use one that begins to open somewhere between roughly 180 and 195 degrees F and is fully open some degrees above that, which is why a normal gauge reading is warmer than many drivers expect. Fitting a cooler-rated thermostat to cure an overheat is a bad trade, because modern engines are calibrated around their rated temperature. We fit the rating your vehicle maker specifies. ## Our thermostat replacement process 1. **Warm-up test:** We watch coolant temperature on a scan tool from a cold start and check hose temperatures. 2. **Confirm the rating:** The replacement matches the opening temperature and design your engine specifies. 3. **Replace:** The thermostat and seal are installed in the correct direction with the bleed valve positioned. 4. **Refill and verify:** We refill, bleed, and confirm a normal warm-up and fan cycle on a drive. ## What's included in thermostat replacement - Cold-start warm-up scan - Upper and lower hose temperature check - Thermostat with the specified rating - New gasket or seal - Coolant refill and air bleed - Code clear and warm-up confirmation ## Is it a need-now or need-later repair? **Need now:** - Thermostat stuck closed or overheating - No defrost heat in cold weather **Need later:** - Slightly slow warm-up with a code but normal driving - Replacing the thermostat at the next coolant service ## How pricing and timing work Some thermostats sit on top of the engine; others are buried under the intake or built into a housing sold as one unit. Coolant is its own line. The diagnostic fee is stated at the counter before testing and applies to approved related repair. Many thermostat jobs finish the same day when the board allows; call before you drive over. ## Aftercare Your gauge should now reach its normal spot within the first few miles and stay there. Heat should be steady. If the gauge still wanders, a sensor may be at fault; our [coolant system diagnostic](https://route66autorepairshop.com/cooling-heating-ac-services/coolant-system-diagnostic) covers that. Check the level cold for a few days. ## Frequently asked questions ### Should I remove the thermostat to stop overheating? No. Without it, the engine runs too cold, and on some engines coolant bypasses the radiator badly. It can cause more heat problems, not fewer. ### Can I use a cooler thermostat? Your engine computer expects a specific range. A different rating can cause codes and poor fuel use. We fit the specified one. ### How long does a thermostat last? There is no fixed life. Many are replaced during coolant or water pump work because the part is inexpensive and the system is already drained. --- Call (909) 608-7699 or visit https://route66autorepairshop.com/contact for a written estimate.