P0420 code

[Madness]

I have a 03 GT n/a. The cat was replaced about 15,000 miles ago as well as the O2 sensors. The car seems to run good, but is recently showing the P0420 code. I am getting 19 mpg city and 26 or so on the freeway at 70 mph. Soon, I will be driving 150 miles day and was wondering if I should just put black tape over the SES light? The car has 115,000 miles and a universal cat was installed at 100,000. Could this be part of the reason why my mileage has dropped off lately? It has a new air filter and fuel filter as of last week. The P0420 is the only code showing up when I do a scan.

Thanks in advance for the replies

[JJSmith]

Does it have a aftermarket cam in it?

[djrepoman]

Code P0420 means "Catalyst System Efficency Below Threshold (Bank 1)"
P0420 is actually Catalyst System Efficiency Below ThresholdCatalyst System Efficiency Below Threshold.
This means that the secondary O2 sensor detects that the catalytic converter isn't working as efficiently as it should.
This engine code can be triggered by any one of the following:

1.) The catalyst in the catalytic converter is cracked, clogged, or otherwise damaged.
2.) The secondary O2 sensor is malfunctioning.
3.) There is an exhaust leak.
4.) The computer unit is faulty.