Code P0420, check engine light is on?

Hello, do you have your own scan tool? There are a couple of live engine data PIDs to look at when the engine is running. Catalytic converters do not fail for no reason, most of the time it's an overly rich air/fuel mixture or contamination from oil or coolant being burned in the engine. But I'll go over the service information on this vehicle first then give you some things to check, most first reaction to a P0420 or p0430 is to replace the Cat, but if the issue that caused the failure in the first place is not resolved, the new one will fail as well.

If you do have a scan tool, what kind is it? Is it just a basic OBD2 code reader, or can it do a full system scan of all the modules in the vehicle? Live engine data is key here. You will need to see what the engine computer (PCM) is doing and how it is adjusting the air/fuel ratio, if it compensating for a lean condition, or there are cylinder misfires occurring or if the PCM is getting bad data from a sensor that's not working correctly, causing the PCM to enrich the mixture, this can cause premature Catalytic converter failure. Cylinder misfires send semi burned and raw fuel down the exhaust, and this can cause issues as well. The Converter is supposed to be able to store oxygen inside to help burn off the emissions gases the engine puts out, that's the whole point of it, to prevent pollution, but when the air/fuel mixture is incorrect it is not able to store oxygen as is should, this can also set a P0420 code. Monitoring the rear oxygen sensor voltage signal can show if the converter is functioning properly.

Another live data point to look at when the engine is running at idle and then also at higher RPMs is the Long- and Short-Term Fuel Trims, they can tell you a lot about how much the PCM is compensating for a potential issue. Positive numbers (ie, +15% for example) means the PCM is adding fuel to the mixture for some reason, Negative numbers mean it is taking fuel away from what it sees as an overly rich mixture.

But skewed sensor signals can fool the PCM into situations where its adding or taking away fuel when it shouldn't be and then codes will start setting. Fords can be difficult sometimes with setting misfire codes, they need to hit certain thresholds to set, and a minor cylinder misfire that has been happening a long time will eventually cause Converter problems.

Here's a list of possible causes that will set a P0420 code, but we should be able to figure out what's going on by scan tool data. Were there any other codes set?

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter