How to Troubleshoot Automotive Thermostats

Troubleshooting an automotive thermostat requires a process of elimination. A malfunctioning thermostat causes one of two symptoms. First, if the thermostat fails in the closed position, the engine overheats. Second, if the thermostat fails in the open position, the engine fails to produce auxiliary heat inside of the vehicle. Unfortunately, other pieces of equipment can cause the same symptoms. On the positive side, ruling out those other pieces of equipment is the best method for confirming a broken thermostat.

Instructions

Engine Overheating

1

Drain the radiator fluid into a bucket and inspect it for sludge. Radiator fluid can be one of several different colors but the most common is green and pink. If the fluid looks discolored or you see a thick sludge buildup in the fluid, clean the inside of the radiator out with a garden hose and nozzle. This is called a radiator flush. Once the radiator flush is complete, fill the radiator with new radiator fluid. Refer to the owners manual but in most cases a 50-50 mix of radiator fluid and distilled water is recommended. With the radiator filled, start the engine. If the engine does not overheat, the issue was with the radiator fluid. If the engine still overheats, move to the next step.

2

Inspect the radiator fan to see if the fan engages after five to 10 minutes of engine running. If the fan does not kick in, this is the reason for overheating. Replace the fan relay switch, as this is the cheapest place to start. If replacing the fan relay does not work, change the fan motor. If the radiator fan started or the engine still overheats after the fan was repaired, move to the next step.

3

Remove the thermostat from the engine. The engine will run without a thermostat, however, it will not produce auxiliary cabin heat. This is a great way to test the thermostat. If the thermostat is removed and the engine still overheats, the thermostat is not the problem. If the engine no longer overheats, the thermostat is faulty. Most people, when putting the thermostat back into the engine after removing it, decide to use a new thermostat in any case, since the part is inexpensive. To remove the thermostat, remove the radiator hose from the location where it connects to the engine. Unbolt the metal cover for the thermostat and then pull the thermostat out of the engine.

No Auxiliary Heat

4

Inspect the floorboards on the passenger side of the vehicle. The first leading cause of auxiliary heat issues is a broken heater core. When a heater core breaks, it often leaks radiator fluid onto the passenger-side floor boards. If the floorboards are not stained or wet with radiator fluid, the heater core is not leaking.

5

Turn the engine on and turn the heating control to full heat. Place the heating control on defrost and set the blower to the fastest setting. After a few minutes, inspect the windshield for a greasy or oily film. If this occurs, the heater core is aspirating radiator fluid into the air vents. The heater core is the issue, not the thermostat.

6

Start the engine and lift the engine hood. Locate the two small radiator hoses that go into the firewall on the passenger side of the vehicle. Each hose is approximately an inch to two inches in diameter. Inspect the hoses for leaks, especially around the bulkheads where the hoses go through the firewall. If any leaks are present, replace the hoses and test for auxiliary heat. If the issue is corrected, the thermostat is fine. If not, the thermostat is faulty.