99 MGM: Catalyst Efficiency Below Threshold

I know that the code "P0430" (Catalyst Efficiency Below Threshold-- Bank 2) probably means that I have a bad O2 sensor BUT, does that necessarily mean that the sensor is BAD... Or could it just be a temporary malfunction?

Check engine light came on about 3-4 miles after my wife fueled the car while it was running. Could that have anything to do with it???

I know that the code "P0430" (Catalyst Efficiency Below Threshold-- Bank 2) probably means that I have a bad O2 sensor BUT, does that necessarily mean that the sensor is BAD... Or could it just be a temporary malfunction?

It means the rear O2 sensor is detecting that the front catalytic converter is not working like it should. It may be due to bad gas that your put in the car although other problems usually arise in that case, clogged/dying catalytic converter, or dirty/dying sensor.

Reset the code and see if it returns. If it does come back, monitor the voltage at the rear O2 sensor. IIRC, it should mostly read lean but some switching to rich is normal. Atleast that's what I was told last I asked about it.

If you have a P0430, that is not indicitave of an O2 sensor problem. The sensors read lack of oxygen and will let you know if the converters are working. As Blue stated, erase the codes to see when it returns. I have yet to cure a P0430 by replacing an O2 sensor. It might delay the converter replacement, but not cure the problem.

Should the catalytic converters be replaced at any particular mileage on Grand Marquis/Crown Victoria (i.e., 50,000, 100,000 miles)? Out of curiosity, what are the consequences of not replacing bad converters?

They should not be replaced unless they need to be.

If you do not replace a catalytic converter that is bad it may cause an excess of noxious gases coming out of your exhaust and you will fail emissions if your state/county requires them.

If the converter is bad enough that it is getting buildup in it and clogged, your engine performance and mileage will suffer greatly.

There are 4 catalytic converters in our vehicles, two two-way cats on each bank. If you have to get new cats, I recommend Magnaflow high-flow 3-way cats. These flow better than stock and you then only require 1 cat on each side to be emissions compliant. Check your state laws before removing cats or replacing with aftermarket type. Alternatively, you can just get them from the junkyard, but you will likely need a torch to get the bolts free .

Oh by the way LieutenantCracker (http://www.automotiveforums.com/vbulletin/member.php?u=568644),
The code you are getting really only applies to the first (upstream) catalytic converter on that bank. The second (downstream) is AFTER the efficiency O2 sensor. Although that doesn't mean it isn't dying too. Usually the cats die from extended rich conditions (i.e. plug not firing, leaking injector) or oil contamination (leaking valve stem seals or rings).