I tried to edit the title but looks like I can't. Real title should be "I just got a P0420 code after truck sat for a week". Once driving again it went out for a week, now it's back on. 99 F150 with 4.6W engine. Not even sure where to start. I did a quick visual check for vacuum leaks. Any tips on how to troubleshoot are appreciated. Truck has 280,000 miles.

 

The P0420 code signals a low catalyst system efficiency. This code suggests that the oxygen levels are below the desired threshold (Bank 1), which most often results from problems with your car's exhaust or fuel systems.

It's probably a faulty connector or O2 sensor.

Or your cats are have been used to their lifespan.