5.1 Testing Oxygen Sensor System (CIS)

The system has four main components: the oxygen sensor, the oxygen sensor control unit, the frequency valve, and the thermoswitch. The oxygen sensor is located in the exhaust manifold, near the connection to the front pipe. The control unit, located in the cowling behind the engine compartment, monitors the signal from the oxygen sensor and controls the frequency valve. The frequency valve, mounted in a fuel line near the airflow sensor, opens and closes at a variable rate to finely adjust fuel pressure, and therefore air/fuel ratio. The thermoswitch, mounted on the cylinder head coolant outlet, interrupts the circuit and places the system in a stand-by mode whenever engine coolant temperature is below 82°F (28°C).

The most obvious indication of system function is the on/off action of the frequency valve, referred to as its duty cycle. To quick-check the system's function, listen for the hum of the frequency valve cycling on and off while the engine runs. Measurement of the duty cycle requires a duty cycle meter or a dwell meter. For more information, see Measuring Frequency Valve Duty Cycle.

The tests should be carried out in the sequence that follows. Testing of an individual component is possible, but without knowing the status of the other components in the system, the results may not be conclusive. Also, testing is completely valid only if idle speed and ignition timing are correct.