1.6 BSE A3 Power Loss - Exhaust/Cat Problem???

[campbell1987]

Hi. We have a 2010 1.6 petrol a3 with BSE engine.
Currently its giving a p0304 cylinder 4 misfire code. We now believe after a lot of messing that it is an exhaust or cat restriction causing the problem or another mechanic suggested possibly an o2 sensor failed.

I have recently bought OBDeleven Pro so i think i could measure live data to test where the problem may be first before pulling the car apart again. But unsure how to use it properly.

Its recently had an engine change as previous one we were told had cracked cylinder rings on piston 4. Now this one is starting to act the same but not as bad yet. Left untreated i think it will end up the same as the old engine.
Im guessing an exhaust/cat restriction caused extra strain on cylinder 4 which over time broke the piston rings on the original engine. (75k miles on it)

Parts replaced so far is
Engine
Injectors
New Plugs
New Leads
New coil pack

Only thing not changed yet is exhaust/cat


Any help or advice here on what i should do to sort it out.

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[Crasher]

The BSE is an 8v engine and they are prone to valve guide wear which causes a burnt out exhaust valve so before you do anything else, get an accurate compression test done.