Error Codes P0400 and P0420
 

I am trying to fix my Grandson’s 2003 Nissan Frontier with a 4 cylinder engine and a 5 speed transmission. When we went to safety inspection in our state to have truck inspected to transfer title the truck showed 2 error codes.
P0400 Exhaust Gas Recirculation Flow Malt. And,
P0420 Catalyst Sys Efficiency Below Threshold (Bank 1).
The errors point to EGR valve and vacuum lines
I ran a vacuum on the EGR valve and it holds vacuum steady for 20 minutes while motor is at idle.
I also ran a smoke test by putting smoke threw the brake booster line and the only smoke that came out was through carburetor. I could find no vacuum leaks.
In order to find out the problem of why we got these 2 error codes does anyone have any suggestions on what else can I do? We cannot afford to take it to a mechanic shop at $125.00 an hour.