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help on checking post oxygen sensor for my 2000 Corolla

Hi All,

I need help to check whether the oxygen sensor is bad or not. Recently I have check light on with P0420 code. After consulting with your guys in this NG, I decided to check the post-oxygen sensor connectivity. First I followed the steps descibed in Haynes repair manual, test the oxygen sensor heater, it reads 14ohms which is between 11-17 ohms, the book says that it is good. Then I tried to test the voltage at the harness side between black and red wires(the book says black and pink wires where there was no voltage existed), it read ~7V. The book says that the voltage should be the battery voltage which is ~13V, my question is what it means? Do I have a bad electrical connection between the harness and oxygen sensor? Next, I did backprobe by inserting a pin along the black wire at the back of the harness connector, then started the engine, it run about 5min, I noticed nothing(the book say that I should read 100-900 millivolts). I am wondering whether I did wrong with the tests. Could someone point out what I did wrong here? How could I make sure that the oxygen is bad before go-ahead to have it replaced? The other thing is: after I cancelled the OBD II code, it took over

2-months for the check light to come back, and I drive 60miles per day to go to work. What happened here? Thanks. wm
Reply to
wenmang
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Without a wiring diagram, it is difficult to provide you with assistance, especially if you are looking for the wire colors. Also, when you refer to "post-oxygen sensor," I am assuming that you are referring to O2 sensor #2 which is located after the catalytic converter. Take a look at the color of the wires to plug into the O2 sensor, paying attention to secondary or "tracer" colors on the wires. Then, find those wires where they come into the ECU. The voltages at the ECU should be the same as at the harness. If they are not the same, then you have a bad connection somewhere, probably due to a bent or loose pin in the connector.

Next, I did backprobe by

When you say that you did a backprobe, I am assuming that you were using a digital volt meter and that it was set to the correctg range. Most likely, the pin you used did not touch the pin in the connector. Try using a thick sewing needle or thin nail to touch the pin in the connector and then concnect your meter to the needle/nail.

The book should have a resistance value for the O2 sensor.

If the O2 sensor is marginal buyt not failed completely, it often takes time for a MIL to reappear.

Reply to
Ray O

Thanks Ray. I did backprobe by using a pin and I measured the voltage between the pin and the harness connector. It reads the same as the one without pin(~7V) and I assumed that the pin did have the contact with connector wire. I did use digital volt meter. Anyway, next step, I tried to loose up the O2-sensor from pipe using a O2-sensor socket. But, nothing. It was so tight that I was afraid to break the threads if I had used brute-force. Is there anyway to loose up the O2-sensor by applying WD-30, etc? The engine was cold though, but still no help. Wm

Reply to
wenmang

I was thinking about your original post. I don't remember what voltages you were getting, but they should be in the 4 to 7 volt range.

As far as removing the old O2 sensor (or any threaded connector that is stuck, spray with penetrating oil, then tap lightly with a hammer. Do this once or twice a day until it loosens up. Working on a hot exhaust may be a little easier.

Reply to
Ray O

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