P0420 code back again

[ptns96grandam]

I have a 96 Grandam 2.4 with 169,758 miles on it. Problem is I got P0420 code back again. Replaced both sensors then cat. Problem went away after replacing cat a year and a half ago. Now i got P0300 , P0420, and P0172. I was cleaning air intake about a week ago might be reason for other two codes. The mechanic that replaced cat said something made cat go bad. Coils, housing ,boots , replaced three years ago plugs replaced last year along with boots again I erased the P0420 code twice now. Engine light always pops on with code when on the highway.Cam gasket housings are leaking and found a little oil in spark plug holes. don't know if this could be the problem causing boots to ground out. hope I didnt ruin another cat. Anyone know what problem could be or has had the same problem

[gobraves00]

isnt that along the lines of vaccum lines as well

[ENGGUY]

Most likely its burning oil and polluting the cat. Time for new, used, rebuilt or overhaul.

[Porteritojr]

If burning oil, woouldn't you be seeing a bunch of smoke from the exhaust?

[ptns96grandam]

I don't see oil burning when i'm driving.and I don't think its the engine half of the miles is highway miles. It still can smoke tires and still has good power. I notice when it starts it smells like raw gas so i looked around engine and smelled gas near fuel presure regulator pulled vac line off and found drip of gas came out. back of regulator looked like it leaked before. Idle didnt change when i pulled vac line. vac line smelled like gas too. i plan on putting a clear tube on it so i can see how bad its leaking. Would regulator cause it to burn rich and ruin cat. priced regulator cost $100 how hard is it to put on?