97 Ford Taurus Catalyst Codes P0430 P0156

Ford 3.0L Vulcan engine, CARB standards. 118000 miles, elevation 5000 ft.
I have been running for some time with a P0430 code (Bank 2 catalyst efficiency below threshold), with no apparent degradation of engine
performance. It passed an emission test one week before this code showed up. I've been reluctant to fix this problem, since it is very likely a high-priced catalytic converter replacement costing more than $1000.
I now have a P0156 code showing also (Bank 2 sensor 2 circuit malfunction). The idle when in gear slowed slightly and had an abrupt stumble when this code showed up today. It ran fine the rest of the way after the event.
Originally, I understood that the catalyst efficiency code had nothing to do with engine fuel dynamics. But now I'm wondering if it is affecting fuel trim, especially with the failed sensor. The bad side of the catalyst probably fouled the sensor.
Should this combination problem be repaired as soon as possible? Is it possible to do engine damage with an incorrect mixture caused by the failed sensor?
Thanks for any input.
GT
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On Fri, 06 May 2005 03:04:45 GMT, "GT"

More likely the bad sensor caused the efficiency code? The efficiency code trips when the inlet and outlet sensors read too close to the same. A cat USUALLY does not fail quickly, and only a week before the efficiency code showed up the cat was obviously alive and well. The second sensor does NOT affect running - the first does.
I'd be checking all wires and connections to the #2 sensor first, and use a Pyrometer to check the cat - should run 75-100 degrees F hotter at the outlet than the inlet after it has "lit off".

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