02 sensor testing trick

[Classic83owner]

I learned a new trick yesterday thanks to ALLDATA. I had never heard of this one but it works really good and it's easier than the test procedures in the FSM. The full test requires a gas anaylizer for complete and acurate results but you can test the O2 function without it.

This is word for word curtesy of ALLDATA:

1) Disconnect the oxygen sensor pigtail and connect a high-impedance VOM with the positive lead on the pigtail and the negative lead to a good ground.

2) Start the engine and let it warm up, if equipped with an air injection system, disable the system so as not to get false tailpipe readings.

3) Grab the disconnected wire that goes fron the oxygen sensor to the ECU with one hand. With the other hand touch the negative battery post or a good ground. The readings should increase on the VOM up to approx. 1 volt and there should be an increase in tailpipe CO.

4) Move your hand from the ground side to the positive post of the battery. Your body will act as a proper resistor and allow a small voltage to the ECU. The VOM reading should fall to almost 0 volts and the CO reading at the tailpipe should go down.

5) If the tailpipe readings acted accordingly but the VOM readings from the oxygen sensor did not go up and down, the sensor is defective.

My car runs rich at all times so I tried this test.

My test results without an anaylizer were as follows:

.675 volts in step 3 with no increase in voltage when I touch ground. (Car running the same as before I disconnected the sensor.)

Steady but slow drop in voltage in step 4 to about .420 volts after 90 seconds or so. Decrease in idle speed (100-200 RPM) and less rich exhaust fumes. Engine smoothed out to a purr.

My conclusions are: Lazy O2 sensor due to either the sensor itself or a poor ground to the sensor. Reasons being that I never seen the rise in voltage and it did not reach "approx." 1 volt, and in step 4 it reacted slowly and would have taken 5 minutes or more to reach 0 volts if at all. I would condem the O2 sensor outright if this were not a 30+ year old vehicle and the fact that exhaust pipes rust pretty good. (Note to self, resistance test the exhaust pipe to good ground.) Could also be a small resistance between the sensor and the ECU causing the slow reaction in step 4.

Lots of variables here but this trick had excellent results in determining if the O2 sensor was working at all and it tells me it's not working well.