Catalytic converter problem? (P420 code)

I'm trying to figure out my P420 code. I'm trying to see if it's my converter before I drop all that money on a new one. From reading everyone's posts, I've learned that the readings off the wires should be variable on the pre-cat and stable on the post-cat. Which wires do you probe for that? In the Chiltons, it shows that black is the ground and the pink is the pre-heat wire from the EGR. So I've tried the other two (purple and tan) to each other and both to ground. I'm showing a steady increase in voltage on both of those wires as it heats up but no fluctuation like described. I stopped there and didn't even go to the post-cat yet.
History on car...140,000 miles and just put a new 50k motor in about 10k miles ago. The original motor blew an upper intake which over heated the motor and toasted cylinders #1 & #3. I nursed it along for a while but the converter certainly was exposed to some serious oil and collant burning for quite a while. So the fact that I'm getting a suspect converter problem isn't that far out of reach.
Thanks for any help.

DTC P0420 is not normally an 02 sensor problem but a result of the rear 02 sensor doing its job by monitoring CAT convertor efficiency. It means the CAT convertor has less than desirable oxygen storage capability and low efficiency. Therefore, I agree you may have a flaky CAT convertor.

On the 02 sensor wiring the purple wire is HO2S hi and tan wire HO2S lo.

Thanks.
I'm monitoring the voltage incorrectly somehow still because I'm still getting just a steady increase (no fluctuation 300mv-90mv like desribed in all the forums) at the pre-cat O2 sensor. I can't establish a bench-mark to compare the post-cat to (to see if it follows suit or stays steady). I've tried the HO2hi wire and the HO2lo wire both to ground and then to each other and basically get the same results...just a steady increase as the engine heats up. I'm obviously doing something wrong. Any suggestions on the proper wire and way to test to get what I'm looking for?

Another way to ckeck for a bad cat is to compare the inlet vs outlet temps at the cat using an infrared thermometer, the outlet should be at least 100-150 degrees hotter than the inlet, fully warmed cat.

Update -

Forgot to mention before, this is a 98' Bonne with a 3800k engine.

I finally got my hands on an infrared thermometer and also figured out what I was doing wrong with checking the H02S wiring...didn't have the ignition on. :screwy:

The temperature at pre-cat is roughly 330 and post-cat is roughly 450. Does anyone know what the "light-off" temperature for this cat. converter should be? (I've read that it can vary from 400 to 600 degrees) Unless the infrared thermometer that I borrowed isn't calibrated correctly, these temps seem rather low to me!

The reason I ask. With the ignition on, I'm getting the appropriate 400mv at both H02 sensors. I start it up and get the appropriate 100mv-900mv (fluctuating) voltage at both sensors. From what I've read, it'll do this until the cat. converter hits it's "light-off" temperature...before that temp, the converter doesn't really do much of anything so the post-cat sensor will mirror the pre-cat.

Unlike a lot of the forums that I've read where people had this same code pop up, my car is actually running a little rough. Idle isn't the greatest (and seems a little lower than usual) and it does seem down on power a little bit.

Any suggestions of what to try next, would be much appreciated!

The temp difference is about right for an operating cat, but the rear o2 sensor still may not be "happy". With an Oscilloscope you shoud see the switch rates of the post cat sensor about 50% of the precat sensor assuming the sensors and cat are good, of course a very lean or rich mixture can skew these readings. Hard to measure the actual cat temp because of the shield/casing. A graphing scanner might give you a better look at the comparative o2 activity. It is not uncommon for coolant to damage a cat. An aftermarket cat is about 185 installed.

Both O2 sensors are less than a year old but were put in place before the whole engine melt-down so they could be toast but they appear to be working within their tolerance.
One thing to note though, the pre-heat wire (from what I've read) should show about 10-15 ohms when tested against ground with the car not running. Both of mine are reading about 3.8 ohms. I'm not sure if this is even a valid test or not though or if the 10-15 ohms would apply to mine.
From watching the multimeter when hooked up to the post-cat O2 sensor, I can see that it switches far less frequently than the pre-cat. It still doesn't stabilize at all though around the 450-500mv like it should. So I'm not sure if while testing, the converter is just not hot enough.
Even at the exhaust manifold, I'm only reading about 530 degrees which seems really low.