2015 Subaru WRX mid-pipe/exhaust replacement

I purchased a 2015 WRX about a year back and recently I had begun to suspect a turbo/exhaust leak as I felt slightly sluggish acceleration and a lack of response from my turbo. Soon after my check engine light came on and the computer was giving a code of “P420” catalyst system below threshold. In bring it to the dealer for my regularly scheduled oil replacements (coincidental) I asked the tech to give it a once over and sure enough… there is a decent sized leak in the mid-pipe from a poor weld. The tech showed me a picture on his phone and it was obviously not a factory weld. I wish I had the foresight to have him send me the pic. The car had been in one minor accident before I purchased it so I am assuming someone did a slap-dash job of repairing it. I’m assuming some recent jolt did it in. The system is otherwise stock.

In lieu of paying north of $3,000 for Subaru to replace just the mid pipe I would rather buy an aftermarket kit and install it myself. Both as a cost cutting measure and to become more familiar with the mechanics of my car. I have attached a picture of the parts list supplied to me for just the mid-pipe assembly below. However, going aftermarket I would like to replace from the damage, back. Which I am assuming means I need more than just a typical Cat-back system. In this case is it actually a Turbo-back system system I need? Or is it a Cat-back + a catted J-pipe? Are those in-fact the same thing and I am just a dummy?

Overall if I can keep the damage to about $2,000 and get a small performance boost I would consider it a win. I suspect something may still be funny with the turbo but as this is the obvious issue at the moment I think it make sense to solve this and see if it solves the other symptoms before moving on to hunting for a turbo leak. Any advice, suggestions, best practices would be GREATLY appreciated.

Thanks in Advance for your time!

@TLAR may be able to SIGNIFICANTLY help you with parts prices.

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is post-turbo, and will not make for sluggish acceleration (or said more properly, I would not expect replacing a factory unit, hole or not, with another factory unit to make much if any difference in acceleration and/or lack of response from the turbo). It might play a role in the P420 (which is an incomplete code; they should have 4 numerical digits, so I’m assuming this is a P0420, and the first zero was typographically dropped).


is also post turbo, and subject to the same caveats as above.

So, in short, I expect replacing all this (with factory equipment) to make zero difference in turbo response, but an exhaust leak must be fixed for state inspection (if nothing else) and all of this should be fairly bolt-on (keyword: SHOULD - rusted bolts and the like can make it an adventure).

And a catted j-pipe + cat back is the same thing as a turbo back.

I’m curious what Tom’s sources say about replacement parts…

And good luck with the project! Wrench turning can be buckets of fun!

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I expected as much as the turbo obviously spools and build pressure but no perceivable boost. Logically if it is getting that far then the problem is that that boost is not being delivered to the cylinder. hence, a pressure leak in the turbo. I have seen some DIY type smoke machines used for finding turbo leaks, any other advice on diagnosing and fixing these or is it something I should have a professional look at?

The thing that is most likely to be helpful is in-person assistsnce. An extra set of eyes and ears, especially some with experience/expertise, will likely be most helpful. Boost leaks should be readily observable…
Regrettably, my in-person time for this month is deployed elsewhere (and you’ll likely get offers for better help anyway).

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Thank you for the advice! I have been involved with the makerspace for a while but mostly in the wood shop. I would very much like to get involved in the auto community here and I am waiting with much anticipation for the next event I can be a part of.

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Best to get it on the lift to see what we’re up against.
Internet diagnosis of automotive issues is sketchy at best, unless there’s a good understanding of terminologies between the doctor and patient.
Going with a turbo back, and a tune is the best way to if you want a performance bump.
Depending on how bad the accident was, the leak may be on the intake side, not the exhaust side.
Let me know when you’ll be at DMS, I’ll take a peek. Best to plan to be there for several hours at best.

P.S. Dress to be ready to get dirty.
Also, if you like it quiet and civil, going aftermarket performance exhaust is not the way to go.


An option for the exhaust is welding up your own. Tools are available at DMS, after all. This will take slightly longer than an off the shelf solution, but more satisfying when done well.


The one item to note is we do not have a proper setup for backpurging the pipe with Argon; so if it is stainless it will sugar. Some have improvised the setup for that here, but if you are using any of our tools for the purpose I request you contact the committee so it can be checked that it’s appropriate. There is an alternative of a paste of some kind, but I’ve never used it myself so I can’t speak to it.

If it’s not a stainless pipe then it won’t matter so much.



It’s stainless. Well, if it’s a proper replacement (or factory) it’s stainless. I forget when the change happened (early 90’s, I think) but practically every factory exhaust became stainless owing to the federal mandates on “emissions equipment warranty”. So although stainless isn’t mandated, nothing else will survive to the mandated time period in the necessary environment (i.e. rustbelt).

SS tubing doesn’t necessarily have to be TIG welded though. And for automotive exhaust work, not that much of an issue from what I’ve seen if you TIG it.

I wouldnt go through the trouble of a stainless exhaust without tigging it.

^^ This. You have a high chance of blowing through with the MIG or other methods anyway, on top of needing to buy a spool of stainless wire and adjust appropriately.
The sugaring would happen regardless of welding mode; it’s on the other side of the weld and just how stainless reacts with air under that much heat.

It’s debatable if sugaring is actually impactful for exhaust purposes; there is a right way to do it, and I’m just informing everyone that we don’t have a setup on hand ready to “do it right”

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Thank you for the offer! I’m open all weekend and I can make it there most weekday evenings as early as 5-6pm. Does 2pm Saturday work for you?

I am more likely to purchase something at this point over trying to fabricate something myself. I took some metalworking classes in college and learned both MIG and TIG welding and wasn’t too shabby at it. However, nowhere near a professional level and only on mild steel. Given the state of the roads here I would sleep better at night with something purchased from a reputable supplier.

After 7pm on certain weekdays is best. This Saturday may not be good.

Get something chambered with a resonator if you like it quiet(er/ish). :grinning: I forget, but the Invidia’s are supposed to be 95db “only” I think.

Not from what I’ve seen(perhaps >500 builds in MIG & TIG), but you are correct on the best practices. Some people might just take it as “not do-able” and not do it at all.