No ODBC1 ECU codes last few times I checked. Only one 02 sensor on this 850 . Well at a minimum I think the cat is shot, so I'd like to know before getting a new cat.

So how can i test for this?

The easiest - I might have the oil analyzed. Never a bad idea.

As far as testing the 02 sensor - I think that runs on a sine wave so something more complex than a voltmeter would be needed to test it. Perhaps the tool called a scope. Does anybody sell a new scope type tool for under $200?

Perhaps a bad temp sensor is telling the engine it's cold. The temp gauge on the gas seems to be accurate. Does the ECU use the same signal.
Lean fuel injector - could be confusing the 02 sensor - perhaps telling the ECU to add more fuel.

The 02 sensor (before the cat) itself is 22 years old with 120K miles and has resisted attempts at removal witha 22mm wrench. Overall it seems fairly accessible. I did try to heat the area with a blue flame - no help. Next thought might be a specific 02 sensor remover. Snap on even makes one with a flex handle.

I did a compression test Saturday. Good numbers even though I forgot to open the throttle.

Any ideas on testing or 02 sensor removal?