P0420 Code. Where to start?

My F-150 (2005, 4.6L XLT, 150k miles) had a check engine for a couple months now. I’ve erased it and it has kept coming back. P0420. I’ve done some reading. Apparently it’s the catalytic converter, O2 sensors, or bad gas. I’m not sure if the cats are to blame as the previous owner swapped the stock exhaust for a dual pipe Flowmaster (40 classic I believe) setup. Ever since my latest fuel up I’ve been getting the worst gas mileage, but that code has come up before so I can rule out bad gas. I’ve replaced the fuel filter recently as well. Should I replace the O2 sensors? Or take it to a mechanic to check for cat problems? Thanks.

Begin by exhausting the converter of its oxygen by inducing a rich mixture, then transition away from this rich state, allowing it to become lean.

Start by depleting the converter of all its oxygen by forcing the mixture to run rich, which means there’s an excess of fuel compared to oxygen.
This process consumes all available oxygen in the converter.
Afterward, transition away from this rich condition to make the mixture go lean, where there’s more oxygen relative to fuel.
This cycle helps in efficiently managing the catalytic converter’s operation for optimal performance.

First, use extra fuel to use up all the oxygen in the converter.
Then, change to using less fuel to make the mixture have more oxygen.

Since you keep getting the P0420 code and you’ve modified your exhaust, you should start by checking the O2 sensors.

If replacing them doesn’t solve the issue, it might be a good idea to take your truck to an expert to check the catalytic converter for any problems.

Your P0420 code could be due to bad oxygen sensors, which are cheaper to replace than the catalytic converter. Try replacing the sensors first - it’s a common fix for this code. If the code persists, then the converter might need further inspection.

Fixin’ that pesky P0420 code on your 05’ F-150, huh? Sounds like you’ve eliminated some culprits (bad gas, fuel filter). @Throttle_Stang

Since the O2 sensors are cheaper than that cat converter, those might be the spark plugs you need to change first

Or Look for your mechanic to check it out for you