Fuel trim and code P0420

Hello, the Fuel Trim numbers are the engine computers correction for any issues that have to do with emissions related components, if for example you have a vacuum leak, you will get positive (+10%) Fuel Trim numbers. This means that the engine computer is adding that %10 more fuel to the air/fuel mixture to keep it at 14 to1. Which is the closest ratio manufactures use to keep the emissions levels down but still have the power output that the engine needs. So Long Term Fuel Trim is a learned %. Over time the engine computer (ECM) will learn where that number is and will use it at every engine start up unless the memory is reset. The Short-term Fuel trim is what the computer is doing right now, so it's more active. We will add up the two numbers and that will be your overall fuel trim.

So right now, you're at a total of +19 on your fuel trim, that's a huge amount. and if it is being caused by a misfiring cylinder(s), yes it can damage the Catalytic Converter. Do the fuel trim numbers smooth out at higher RPMS? If they do, then most likely you have a vacuum leak. If not or they get worse, then you may have a sensor issue or something else that's causing the engine to run lean. I say Lean because the ECM is adding Fuel right now to the air/fuel mixture. The code went away because the ECM is just not seeing the problem right now. We are going through a switch over to Summer Fuel mixtures right now and that may be why the situation has changed a bit. Do you have any other codes besides the P0420?

https://www.2carpros.com/articles/how-emission-control-systems-work

Does the Fuel Trim smooth out closer to 0 at higher RPM?

Thats pretty normal for a rear oxygen sensor, especially since these are regular narrow band oxygen sensors, the rear should be pretty stable. What I was asking is the Short Term and Long-Term Fuel trim numbers. Do they come down from +13 and +6 to around 0 or close to 0 when you raise the RPMs? We need to know what the fuel trims are doing before analyzing the Oxygen sensors.

Yes, the Purge Valve is pulse width modulated, the ECM does this to control exactly how much fuel vapor is entering the engine, it will purge the Evap system at different times depending on a number of variables. But since your fuel trims get better while you're driving it sounds like you have a large vacuum leak. I say this because when the throttle plates are closed, such as at an idle state, the intake manifold has high vacuum. When you're driving and the throttle plates are open or partially open, this lowers the manifold vacuum. This is why at idle a vacuum leak will be worse, because there's more vacuum and more air is being drawn in due to the difference in pressures outside the engine and inside.
When the throttle plates are open, manifold vacuum drops and less air is drawn in, so the ECM has less to compensate for. This is called "False Air" because it is not being measured by the ECM.
This car has a Mass Air Flow sensor (MAF), and the vacuum leak wherever it is (i.e., intake manifold gasket,) the MAF cannot measure that air coming in and your fuel trims go positive because the ECM is adding fuel to make up for the extra false air.
So, at idle is when you're going to want to search for the vacuum leak. The +1.6 and +3 are great numbers.
Here is a guide below to help you find the leak. Look for old vacuum lines that could be broken or rotted apart. Try blocking off the Brake Booster hose that goes into the intake manifold.
i usually use a smoke machine to find vacuum leaks, but I also use a spray bottle with just water. If you spray around the intake manifold gasket area and listen carefully you can hear the water being sucked into where the vacuum leak is.
Some people will use a propane or brake cleaner and listen for the rpm to raise when you're around the leak. But I prefer to start with water, not a flammable spray. Thats a last resort.
Watch your fuel trim numbers, if you can find and block the leak, the trim numbers will come down pretty quickly.
If you suspect the Purge valve issue, just unplug it, or block its hose to the intake manifold temporarily. See if that helps. Let us know how it goes.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

There are many manufacturers that have issues with intake gaskets. GM/Chevrolet with their plastic intake manifolds almost all leak, those are really easy to find. You can hear them it's so loud. Since the age of your vehicle though it could be many things. Even a valve cover that's leaking oil. Is a vacuum leak because of the PCV system. Check the Air Intake boot that goes to the throttle body for cracks too. Since its past the MAF the ECM will not read that incoming air.
One other thing to remember is that the Oxygen sensors read Oxygen, not fuel. So, if you have misfiring cylinders, that unburned air and fuel is going down the exhaust and the Oxygen sensors will only read the air, this can send fuel trim numbers positive too.
When the sensors read a lack of air the ECM will interrupt that as a rich condition and the fuel trim numbers will go negative. Just things to keep in mind.
If one of the front Oxygen sensors is stuck at a low voltage (below 0.45v) the ECM will add fuel during that condition. The Oxygen sensors should be active, swinging voltage from around 0.2-0.8volts constantly. If a front one (before the converter) is stuck low, that will cause high positive fuel trim numbers.
The rear Oxygen sensor is really there to monitor the converters efficiency.