After driving the car for about 100 miles after the service work, the check engine light came back on, and it is throwing a P0420 code on catalytic converter, however jut before that it pass Ohio Emission tests, and the code when away after vac leak was fixed. is it possible it is an o2 sensor.

The following parts were used to fix the Vac Leak and code issues.

20 Ford FOPZ*9229*A o-rings for the Injectors and fuel rail
1 722*97422* injector pigtail
6 XR843533 Gaskets Napa
1 96494 Gasket Jag lower Plenum
1 Motorcraft Battery

A total of $263.00 in parts.

 

Mine is showing the same code. I'm not sure yet what's causing it. I had a coil go bad and the engine misfired on the drive home. Could be that I damaged the cat.

I tried resetting it, but it comes back fairly quickly. Maybe 2 starts before the code reappears.

 

P0420 is on Bank 1 with several possible causes other than the cat..

 

If you had the P0420 code suddenly pop up after the mechanic had the manifold off, it is possible that during reinstallation of the manifold that it was not done correctly and there is now a leak from improper installation of the upper set of manifold gaskets for the plenum (the part number that you gave is for the upper, not the lower gaskets).

Also there is a possibility that there is a leak in the IMT o-ring gaskets causing an intake manifold leak.. if you look on the passenger side of the intake manifold, you will see 2 round covers secured by two bolts and the wiring harness clipped in (these are the IMT valves). These are sealed with an o-ring to prevent an intake leak.. as the vehicle ages, these can become dry and brittle causing a failure of this o-ring gasket.. they are inexpensive and easy to replace (the top one, you can remove the IMT valve and replace the gasket. The bottom when you unbolt the IMT valve, you cannot fully remove it because of clearance issues, but you can simply break and remove old o-ring and carefully stretch new one over backside, without damage, since new and elastic). this tip for the lower IMT valve will save you the headache of removing the intake manifold.. I would replace these o-rings, First, before removing the intake manifold if you want to check the upper plenum gaskets, if replacement of the IMT o-ring gaskets does not fix the code. See the image that I posted for other possible causes of P0420 before you jump to the expensive conclusion of a cat problem..

 

I forgot to mention that it may even be something as simple as someone not torquing the manifold bolts on correctly and leaving them loose.. also, three of the bolts on the manifold are different lengths and need to be put back in the exact same place or can cause a leak..