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Discussion Starter #1 (Edited)
I used soapy water last weekend to check for leaks along my entire exhaust system. Pushed air through the catback with a shop vac hooked up in reverse (damn Tomei exhaust never seals with those slip-fittings and stupid clamps!)

Decided to test the exhaust manifold the same way, but I cold-started the engine and as quickly as I could sprayed soapy water all over the gasket joints of the manifold. I found what I think is a cracked O2 sensor, because it did blow bubbles and not just sizzle from heat. I could feel exhaust gases pulsing out around the sensor too.

I was planning to reseat the sensor and check for leaks again in case I installed it poorly, but is there anything else I should check before outright replacing the sensor? I've been hunting down the cause of my weird fuel trims as well as loud tapping noise at cold-start and I think I found it.

In terms of cause, some of you may have seen my other posts about how I'm fixing the work from previous shops installing a KB oil pan and IAG AOS for me. I've found cross-threaded bolts, leaking gaskets (exhaust, oil pan), etc along the way. My guess is they removed the O2 sensor and possibly dropped it, or maybe over tightened it during reinstall? I'd be happy to admit if I broke something, but I've handled the O2 sensor like an egg and use the factory torque specs when reinstalling.
 

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Discussion Starter #2
False alarm I guess, I reseated the sensor and it seems to have sealed up. Did another cold start leak test with soapy water and I found zero bubbles. Fuel trims stayed tidy within -8% and 0% for a 15 mile drive too.

Btw, if anyone needs a Denso upstream O2 sensor, Amazon has the best price that I found, beating out every car parts store I could find by at least $30. I think you still get the 1 year parts warranty too from Denso, still waiting on confirmation about that.
 

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Figured I might help this thread stay useful since I still have no issue with the reseated O2 sensor:

I received confirmation from Denso that they do NOT honor Amazon as a certified parts retailer. So if you are looking for the parts warranty and don't want to spend $200 at the dealer, then buy from Densoproducts.com, which is like $3 more than amazon's listing at the time of this writing. Denso's response quoted densoproducts.com as a qualified retailer, and specifically stated Amazon is not qualified.

Amazon says they'll refer warranty claims to the manufacturer, but, per Denso themselves, the manufacturer won't honor claims on Amazon purchases.

Since I never said this anywhere else, the 2016 STI (I believe this applies to '08+ cars as well) upstream O2 sensor part # is 234-9034 from Denso.
 
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