Testing Oxygen Sensor Function

This is a sensitive measurement. In the interest of accuracy, the engine must be fully warmed up, with idle speed, ignition timing, and CO content adjusted to specifications. The exhaust system between the engine and the catalytic converter must be free of leaks, the engine coolant temperature sensor should be connected normally, the voltage supply to the oxygen sensor heater must be OK, and all other electrical consumers including air conditioning should be switched off. In addition, test measurements should only be made when the radiator cooling fan is not running.

Run the engine and record a baseline CO measurement. With the engine running, raise the engine oil dipstick slightly. This allows extra air into the closed crankcase ventilation system and, therefore, into the intake manifold. If the oxygen sensor is performing correctly, the CO reading should drop for a few seconds, then return to the original value.

To check oxygen sensor wiring (Digifant II only):

1.	Disconnect the harness connector from the Digifant control unit, as shown in Fig. 5-7.

Fig. 5-7.

Digifant control unit and oxygen sensor showing mounting and harness connectors.

2.	Using an ohmmeter or a multimeter set to the 20k scale, check continuity between terminals 2 and 13 of the Digifant control unit harness connector. See Fig. 5-8. There should be no continuity. If there is, there is a short to ground in the oxygen sensor wiring.

Fig. 5-8.

Digifant control unit harness connector terminal identification.