EJ25 Engine Problems

OK so I have a 1998 Legacy GT, it's a 2.5 5MT sedan with 185k miles and I replaced the original engine a few months ago. The current engine is the same 2.5 as my original just from a different car. The original had ported and polished heads and intake manifold, headers, high flow cat, stainless Stromung exhaust and an Injen CAI, which I swapped on to the current engine. The car has had the check engine light on since I bought it but it's just an o2 sensor code and isn't a problem, but a few weeks ago my cat blew out and I replaced it with a Magnaflow cat and now it spits, sputters, and backfires whenever I hit WOT, very similar to when a car is out of gas or is getting no spark. I have shitty plug wires atm which are getting replaced soon, but I think the problem is because I have a chunk of converter honeycomb stuck in my muffler, but I'm not 100% sure. Could it also be a bad o2 sensor and if it is, would getting a wideband rear (after cat) sensor, like something from Innovate, do the trick?

You might have something plugging the passage, but also the factory O2 wasn't designed for that flow plus those cats might burn hotter than factory... I know some of the Honda guys run spacers for their O2 sensors that dont let them sit as deep inside the piping as they do from factory. Something to keep the mixture more lean because I think the reading will seem too hot ( lean ) and want to compensate by dumping alot of fuel... could be the issue.. I would look into a O2 spacer. Know what I mean?

i thought you said your check engine light was on because of the o2? sounds like you clogged the cat from running rich. pick up some o2 senors on ebay or amazon denso made the oem ones. go to a parts store and have them check if you have any new codes. you can check your plug wires and see if they are shorting out. they will look like little whiteish marks, or run it and look under the hood when its dark. also you can get a bluetooth obd 2 scanner for real cheap on ebay and poke around and see what is happening with your fuel trim and o2 sensors.

after swapping the engine and the exhaust, and the intake? i would bet on a leak. probably in the intake, maybe the intake gasket if the manifold was removed.

the p0420 trouble code is usually NOT a cat code. there are a lot of things that can cause them, including carts and o2 sensors. but leaks can cause them as well.

A faulty o2 sensor can def cause those running issues. I would hook up a scan tool go into the data and see what the voltage the bad o2 sensor is putting out. A good one @ idle should jump up and down in voltage at a rapid rate. Then if you hold the throttle at 2500-3000rpm it should smooth out.

Just got it fixed. Had a buddy of mine who is a Subaru mechanic help me look at it and it was just corrosion on the plug terminals to the ecu... I also changed the plugs and upgraded the wires to a set of 8mm which helped a lot.