O2 Sensors - Avoiding Bogus Test Results

90mercedes04
TO: OUR MERCEDES-BENZ PASSENGER CAR DEALERS

DATE: May 1990


SUBJECT: O2 SENSOR REPLACEMENT ENGINE 103, 116, 117 STARTING MODEL YEAR 1986 ENGINE 102 STARTING MODEL YEAR 1987
REF.NO. MBNA 07.3/11
In a recent test of returned warranty parts, almost 60% of the tested 02 sensors were not defective.

It is therefore important to check the sensor and sensor circuit according to the O2 Sensor Signal and Heater Test in the Diagnostic Manual.

Additionally, we would like to point out that it is very important to check the following before testing and/or replacing an O2 Sensor.

1. Before testing the O2 Sensor, make sure that the ground connection is OK. Loosen and retighten the O2 Sensor. Retighten the exhaust flange and manifold and check and clean, if necessary, all ground connections between engine and chassis.

2. Do not test O2 sensor after a long engine idle period.

3. Make sure engine is at operating temperature.

To avoid false readouts, a tester with an input impedance of greater than one megaohm must be used (Sun DMM5, Fluke 23). There are many testers on the market which are not up to this standard and can cause false readouts and unnecessary replacement of oxygen sensors.