Does anyone know of any tests that you can run on an 02 sensor to see if they are working at full capacity?

Full capacity? don't know what you mean by that. Sensor output can be from 0 to 1.0 volts, and can be measured with a digital voltmeter. In normal, closed loop operation, the reading will bounce all over that place. I think most of the fancier scanners can read this output out of the computer, as can my holley projection.

I guess my question is I'm wondering if my 92 sensor is working. Gas mileage is about 10 MPG on the hiway and everything appears to be OK but I feel I should be getting better mileage. It's a used vehicle 350 with port injection. Just trying to come up with an idea to get better mileage.

If you have access to a scanner, like Jim mentioned the reading will fluctate rapidly on a good sensor. An error code will flash if the sensor stays high (.7 volt or more) or low (.2 volts or less) for more than 50 seconds.

If you have a digital volt/ohm meter you can measure this on the car. Another neat little tool I bought for mine is an adaptor from the aftermarket corvette suppliers. (like Midway)This plugs into the harness and allows the sensor to plug into it, that way you receive no error message.

Start the engine and let it reach operating temp. Shut it off. Disconnect the pigtail from the O2 sensor. Restart the engine.(you will flash an error, don't worry, you will clear this later.)Let it run for a minute or two and then take voltage measurements from the sensor. Take a continuous measurement for around 1-2 minutes and record the voltage output. Shut off the engine. Reconnect the O2 sensor. Disconnect the negative cable for about 30 seconds (to clear the error) and reconnect the cable.

If your voltage readings are sluggish, then it's a good sign that the sensor is starting to fail. If the reading fluctuates rapidly, then it's time to start looking elsewhere.

How many miles are on the vehicle? An O2 sensor should be replaced about every 45k miles.

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67 Camaro LS6 454/TH400/12bolt 3.73
1989 TransAm 5.7L WS6 W/all the options

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There is 52000 on the vehicle. It runs good and doesn't seem to have any loss in power and I'm only turning 2000 RPM at 75 MPH so I know it's not geared low. Just can't figure any reason for the poor mileage. The tail pipe has some residue showing that it's running rich. The only thing I could come up with, with out any computer to check everything, was the 02 sensor.