An apparent oxygen sensor problem may not be the sensors fault. An air leak in the exhaust manifold, a fouled spark plug, or other problem in the ignition system may cause the oxygen sensor to report a false lean condition.

When checking the oxygen sensor it is important to remember that a good sensor produces a fluctuating signal that responds quickly to changes in the exhaust oxygen content. To check the sensor you need a *10 mega ohm digital voltmeter. Never use an ohmmeter to check the oxygen sensor and never jump or ground the terminals. On a later style (heated) oxygen sensor, be sure you connect to the signal wire and not one of the heater or ground wires.

Start the engine and let it idle. Observe the reading on the voltmeter. With the engine cold and in *"Open Loop" it should be fixed at approximately 0.2 volts. Allow the engine to warm up and enter *"Closed Loop" operation. This period of time is about 2 to 3 minutes.

NOTE: While in closed loop, the meter should fluctuate between 0.1 to 1.0 volts. If the oxygen sensor is slow to respond to the closed loop mode, the sensor is not operating efficiently and is therefore termed "lazy". If the oxygen sensor waits more than 1 or 2 minutes after closed loop operation (3 to 4 minutes total) replace the oxygen sensor. Be sure the engine is at operating temperature and there is not a problem with the thermostat or cooling system.

Further test the oxygen sensor by removing a vacuum line and observe the readings with the lean mixture. The voltage should lower to about 0.2 volt. Additionally check with a rich mixture by either partially choking off the air supply or adding a small amount of propane to the intake charge. This should make the voltage reading increase. The oxygen sensor should not exceed a range of 0.1 to 1.0 volts.

Glossary:
* 10 mega ohm digital voltmeter - A highly accurate multimeter that uses low voltage integrated circuits and a digital display rather than a needle movement.

* "Closed Loop" - The mode of operation a system with an oxygen sensor goes into when the engine is warmed up. An oxygen sensor will not produce accurate voltage until it reaches ~600F degrees. At this point it can send a varying voltage signal to a control until, which alters the air/fuel ratio as needed.

* "Open Loop" - An operational mode (as on cold startup) during which "default" preprogrammed values in the control unit are used to determine the air/fuel ratio, etc.