Possible bad o2 sensor??

Hello again, i just recently posted a message about my "bad cat". Well turns out i don't think it is a bad catalytic converter, but a bad downstream o2 sensor. Someone tell me if i am thinking about this the wrong way. The code that came up was p0420, or "catalyst efficiency problem" i know there are codes for the 02 sensors, but my mechanic buddy has seen sensors go bad without putting out a code. When i tested the voltage of the upstream and downstream sensors, the downstream reading was actually higher than the upstream. I think the upstream sensor was giving a voltage of .65 volts and the downstream .75 volts. this makes no sense to me being that the voltage should be lower downstream or if the cat is completely shot, it should be the same. But higher? seems like a screwed up sensor. So i hooked up the old tailpipe emissions machine and it would have passed. this tells me that the cat is fine and its the downstream sensor that is not working properly. Which still sucks because the sensor costs $114.00. But its better than a bad cat and its way better than dealing with the A-hole car dealership. Sorry this is so long, I just want someone else's opinion. Thanks, Erich.

When the HO2 sensor is at operating temperature, the voltage output of the sensors is 0.45V at the correct Fuel/Air ratio. Higher voltage than this indicates a rich mixture. The higher, the richer. Lower voltage is leaner. The output voltage of the HO2 sensors should oscillate around 0.5 V while operating normally.

The voltages you found (if they were mean voltages) mean that the oxygen content has gone down by passing through the Cat Converter. Since the catalyst is an oxidation catalyst (using up oxygen), this makes sense. If they were peak voltages, then it is meaningless. Peak voltages are a function of the frequency and the O2 content.

The comparison made by the ECM is not relative voltages (since they constantly change owing to the design of the HO2 sensor), but waveform frequencies. The frequency and peak voltage of the upstream [bank 1 sensor 1] sensor should be higher than the frequency and peak voltage of the downstream [bank 1 sensor 2] sensor. If the frequency is pretty much the same, then the catalyst performance has deteriorated.

The HO2 sensors are tested by measuring their resistance. The resistance between terminals +B and HT should be 11-16 ohms cold, or 23-32 ohms at operating temperature.

You should be able to measure the frequency of the waveform from both HO2 sensors (thus checking catalyst function) by connecting a voltmeter (0-1 V scale) or oscilloscope to terminals OX1 [bank 1 sensor 1], OXS [bank 1 sensor 2], and E1 of the ECM connector. Check the terminal voltages/waveforms while running at 2500-3000 rpm. The engine should be first warmed up at idle until hot, and then heatedby running it at 2500-3000 rpms for 3 min.

Note: This discussion does not apply to an engine with an Air/Fuel ratio sensor instead of an HO2 sensor.