test oxygen sensor (code P0420)

I have a 1998 Chevy Venture. The service engine light has been on for about two months. I took the van to Auto Zone and they connected a scanner to the OBDII connector. The scanner showed a code P0420 which seems to be
either a bad catalytic converter or bad o2 sensor(s). Can someone give me some instructions on how to check the oxygen sensors? I can replace the rear sensor, but the sensor near the engine looks like a bad situation however. No room to work, not sure how to get to it to even check it, let alone replace it. But at $67.00 I want to know they are bad before I buy new sensors. If they can be checked and are good, I will take the van to get a new converter.
Thanks, Terry
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Terry wrote:

Hi Terry...
According to a Haynes manual...
Warm the engine up to operating temperature. Shut it off. Unplug the sensor at the connector. Connect the positive terminal of a high impedance digital voltmeter to the sensor side of the connector, negative to ground.
Cables well away from hot or moving parts. Start the engine again. Voltage should be between .1 and 1 volts, and should be constantly fluctuating.
If not, the sensor is probably bad.
Take care.
Ken
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While that is a rough check and will indicate if it is completely bad it could be slow switching etc. Have someone check it with an OBDII tester. If you are going to take it to someone to replace the converter why not just take it to them and have them analyze it. It will be cheaper than swapping parts using primitive checks....

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