[murderspice]

Fully stock, unmodded car. I had a money shift earlier this year that resulted in a full engine replacement by the dealer. Since the replacement, I have had a CEL for the cat -which the dealer is telling me will require a cat replacement to fix. I decided to get an second opinion at a local independent shop, where the following two error codes were logged:

180001 - Catalytic converter: efficiency below limit value (intermittent), and
120308 - Charging pressure control, plausibility Pressure too low (intermittent)

The local shop visually checked and found nothing wrong, but found that the car had an older integration, so updated it and cleared the codes (while charging me nearly $400...(secondary question, was I gouged?)). Two days later and the CEL for the cat has popped up again.

Having just moved across country, I sadly do not have the necessary tools to properly investigate this myself, so I am at the mercy of using the dealer or local shop to diagnose/fix this. That said, I am humbly asking for any advice as to the cause and solution to what could be wrong before I dump more money into testing/replacing stuff.

1. Assuming the cat is damaged, would continuing to drive the car cause damage to the turbo (as indicated by the 120308 code)?
2. What is the most reliable/accurate way to diagnose what the actual problem is with the car? IE, could one of these faults be causing the other?
3. With limited tools, what can I do myself to diagnose this problem? I have done a visual inspection as best I can and saw nothing obvious.

Any help is appreciated!