I did a quick Search and couldn't find this already mentioned, so sorry if this is a repost.
I seem to get a P0420 every 12-18 months, so this recently-release TSB caught my eye. Evidently a PCM update may address some spurious cat efficiency-related CEL's.
Quote:
TSB 04-15-16
08/09/04
MALFUNCTION INDICATOR LAMP (MIL)
ILLUMINATED WITH DIAGNOSTIC TROUBLE CODE(S)
P0420 AND/OR P0430
FORD:
1997-1998 Contour
MERCURY:
1997-1998 Mystique
1999 Cougar
ISSUE
Some 1997-1998 Contour/Mystique and 1999 Cougar vehicles equipped with the 2.5L engine may exhibit a malfunction indicator lamp (MIL) illuminated with diagnostic trouble codes (DTC) P0420 (Catalyst System Efficiency Below Threshold - Bank 1) and/or P0430 (Catalyst System Efficiency Below Threshold - Bank 2).
ACTION
Reprogram the powertrain control module (PCM) to the latest calibration using WDS release B32.5 or higher. Calibration files may also be obtained at www.motorcraft.com. Replacement of catalysts is no longer necessary to address the most common causes of this condition.
WARRANTY STATUS: Eligible Under Provisions Of New Vehicle Limited Warranty Coverage And Emissions Warranty Coverage
Basically, if you have a 1997 or 1998 V6 and the CEL comes on, have the code read for free at Autozone or by a friend with a scanner. If you have just P0420 or P0430, call the dealership and ask if they have this TSB. 
