Glad to help. Let us know what route you go and your experience. I hope it is warmer down there than it is here!

I've had the "check engine Light" turned off to buy me time. And I will go for the Car Inspection that is due end of this month! If light comes back on, probably get 30-60 days to fix problem. After all the research, I will buy the front manifold piece made by Bosal (sounds very solid, costs more but less than best OEM price) and purchase at Rockauto.com. They have best price and shipping and I have a 5% discount code. I've reviewed all others and compared to Toyota OEM-- I found one OEM dealer selling it for just a little higher price, but shipping is way more, and Toyota warranty is 1 yr. Bosal/Rockauto provides 5 yrs. The car is a 2003. 5 yrs. will be great for me.
My mechanic has been very helpful and willing to let me buy the part. I will followup, especially if the problem is the manifold or not. I can't know for sure due to both parts on Bank 1, as I understand. Hopefully it works, otherwise I will be dealing with the converter pipe area under car. To follow....

Typically if P0420 it is first converter which is the one you are looking at.

Suggest asking your mechanic about the smog test procedures in your state. Typically the procedure is to connect a computer to the cars ODBII system and check for set trouble codes (turn on the light), pending codes (test failed once but needs to fail again to turn on light) and uncompleted self checks of various system and components.

In doing self checks, if all systems are OK, sooner or later the computer completes most self checks, but not all sometimes due to the manufacturers computer programming. Most states allow a certain number of uncompleted self checks perhaps two.

In resetting the check light the computer will now self check all items which could result in a lot of checks not being complete when the smog test is done. This could cause the car to fail the a smog test.

The test requiring a cars computer to have completed all or most of the self checks to pass.

I'm just seeing this last post. Thank you, Joey. And you have entered a topic that I do not understand but will see that the mechanic is taking care of this.

So, I was able to pass inspection. Light remains off. But while inspecting, tire wearout came up. And realized my oil change people (combination of Toyota dealer and a Monro shop) have neglected to rotate the tires, and this is included in oil changes.! I need to take care of the front tires quickly. Exhaust Manifold will follow.
Interesting and disappointing development...

Given the car passed the test, wait until the light comes one again with same code. Who knows, maybe it will stay off.

I hope that you find the diagrams you needed!