Checking Thermoswitch

The thermoswitch inhibits the oxygen sensor system when the engine is cold and the sensor signal is unstable. A faulty thermoswitch may also prevent proper operation at normal operating temperature. To check the thermoswitch, remove the wiring connectors and test for continuity between the two terminals on the switch. The thermoswitch is mounted on the cylinder head coolant outlet, as shown in Fig. 5-2.

When engine coolant temperature is below 63°F (17°C), there should be continuity (switch closed). At normal operating temperature, above 82°F (28°F), there should be no continuity (switch open). A thermoswitch which fails either test should be replaced. To continue testing the oxygen sensor system, go to the next test.

Fig. 5-2.

Location of thermoswitch on cylinder head coolant outlet. (Wire connectors 2 shown disconnected from switch terminals 1 and bridged for later test.)

To replace the thermoswitch, allow the engine to cool. Use a wrench on the large hex portion of the switch. Install with sealing compound on the threads, and use a new sealing washer.