Go ahead and take her into the dealer! I just had the same problem and my 97 GL and it is getting repaired today. I was a little worried myself about the possibility of some bogus charges. I was only getting the 420 code, as well, and all O2 sensors were working according my my OBDII software - nothing to worry about, but I've dealt with shady dealers too often. Fortunately, I've had no problems with the dealer (so far) - took it to a Lincoln dealer since I can't find a good Ford dealer around this area - the diaged the problem, ordered the parts, and gave me a call last week to let me know the cats were in. Of course if they mess anything up I'll probably be ranting about it on this board...
Anyway, good luck!
