out driving tonight and CEL popped on. Quick OBDII scan and p0420. Attached is a screenshot. I really hope I do not need to replace the catalytic converter but I feel like that is the issue

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I will add a few notes:

it has spent most of the last 6-8 months sitting, very little driving.

I had the MAF cleaned right before I stopped driving it along with new spark plugs

Last week I took it to the mechanic and had the following done:

Knock sensors and harness
Spark plug wires
Heater hose
All three serpentine belts
Radiator coolant hose
Timing belt
Water pump
Radiator and new antifreeze
Trailer wiring harness
Camshaft seal
Intake manifold gasket
Thermostat and seal
Valve cover gasket
PCV valve and grommet
Radiator cap
Couple oil cooler hoses

Prior to tonight, it has been driven probably 10 miles since I picked it up. Tonight I drove it for 15-20 minutes before the light came on

So I was thinking maybe I drive this old tank of gas out and fill up. Should I bother adding something to the tank such as cataclean? Should I take it back to the mechanic Monday and have them look at it?

A quick way to test your catalytic converters is with your nose. Start the car, let it warm up and go out near the exhaust pipe. If the cats are failing, they will start to allow NOx, hydrocarbons and O2 and other gases to pass through. The easiest ones to smell is NOx which smells like skunk spray and rotten eggs and hydrocarbons which smell like fuel. If you smell that then yeah, your cats have failed.

If not and just smells like plain 'ol car exhaust the rear oxygen sensor is all that's needed. Very simple fix and you can find your replacement sensor here: Find My Part.

The work done by the mechanic should be checked for vacuum leaks etc. Also check your exhaust for small leaks around the cat, or near the downstream O2 sensor.

Or could be like mine with new cats/02 sensors that never hits a P0420 Until, you drive it slowly (5-10 mph, usually at deer lease) for a couple miles, boom every time like clockwork.

Reset and can be driven thousands of miles until....so maybe Nothing really at all.

If you don't believe my scenario, google it, lots of others can replicate P0420 the same exact way, mall cruisers too but didn't wanna' throw them under the bus....