1 Lucky Texan

have a parts store read the codes (or, if you own a smartphone, get an ELM327 BT adapter and use an app like Torque to read them yourself) , the CEL is trying to tell us 'something'. Record the codes, have them clear the codes if they will or do a battery disconnect reset, post the codes here, then, scan again if the CEL comes back on. That way we may see what code(s) come back most quickly. do not drive the car if the CEL is flashing.

what did the old plugs look like? carbon from running rich or ????? backfiring can sometimes point to valve timing or a sticking valve. some older soobs have crank sensors that get temperature sensitive....

check engine light on? how old are the plugs? any sign of oil on the plug wire boots? is it worse in damp weather?

severe miss will pump too much unburned fuel thru and destroy a cat. Something as simple as an exhaust leak can throw a P0420 code - but that wouldn't destroy a cat. many people swap out perfectly good converters when they see 'catalytic converter efficiency low' or w'ever is part of the definition for P0420 - BUT, it often ins't a bad cat. vacuum leaks, dirty MAF sensors, misfires, exhaust leaks, bad wiring, bad rear O2 sensors - many things can throw that code. on a used car with questionable background and present problems - best to start with the assumption that EVERY system needs to be examined for catch-up maintenance and possible repair. Examine and refresh basic consumable parts like plugs, plug wires and filters, most of the fluids, inspect important rubber components..., then maybe get some live data to see where any other issues may be.

steering felt OK? mostly likely alt. You could get the original rebuilt locally if you can do without the car for a few days.

triple check crank sprocket timing - you never use the arrow/reiangle on the front, use the dash/grooved 'tab' at the rear for TB alignment, then do a tooth count after rotating the sprocket 2 times. also, check the large hose from the IACV to the intake tubing connection. ground connections are also critical.

18 year old car? yeah, I'd try the conditioner do you know if the car was overheated by a previous owner? how severely and how often it was overheated? it's possible to install new HGs and have a rod bearing go 3 weeks later if the car saw extreme heat. I know plenty of other people would just go to the headgaskets and that's fine, seems clear from your diagnosis that is what it needs, but the conditioner is only 6 oz or so in the entire volume of the coolant, it was made/selected for exactly the reason of externally seeping headgaskets, it happens to work on other small leaks, it could be flushed from the cooling system anytime you want and, if no other treatment has been used by previous owners and you install 1 bottle only per directions, I've never read of it causing a problem. If anything, it might get you thru winter and plan on new headgaskets when the weather is better. It's cheap too. I havent wrenched very deep but, if I were gonna do headgaskets, I'd do both sides. If I wasn't confident the engine hadn't been overheated, I might drive it while doing the HGs on an engine from a wreck - better chance it wasn't overheating when it crashed. just some thoughts, hope one of the experienced guys will chime in.

kinda wonder if you have a bad ground connection or wire harness/connector somewhere. any sign of rodent damage?

knock sensors crack, go bad, sometimes no code - they cut power by pulling timing, many folks use the cheap ebay units with good results check for slipped cam/crank timing? rare but possible.

coupla ideas get a pressure test kit - some parts store will 'loan' them out. If you still can't find the leak, clean the car up at the car wash, put some dye in the coolant, drive around for a while, use the UV light to search for a leak. maybe consider using some Subaru Coolant Conditioner, its a repackaged leak stop and some folks reduce/cure drips and small leaks with it.

ooohhh, excellent point! maybe he's caught a loose pulley before the bolt has backed-out ! some older instruction for torqueing the crank pulley listed a value that is too low - maybe it just needs to be tightened!

that is a lot the crank pulley is 2 piece with rubber 'laminated' between parts. Mark across the face with liquid paper or yellow crayon/grease pencil or w'ever and see if the outer is slipping.

dunno about Forester, on 'another forum', folks are getting double-digit percent under MSRP on Outbacks. 10-12% under seems 'common'.

i think they haveproper stuff at the parts store - maybe a rental of a light and goggles too. I have only used premixed r134a with dye - found a bad a/c hose.

maybe UV dye would help? I once read of a throttle body with some odd crack that allowed coolant to be sucked into the engine!

well, now I'm going with; something stuck between rotor and brake shield/bent shield, wheel bearing, or brakes. lol! always a good idea to triple check lug nuts too.

could be a dragging brake pad - maybe caliper stuck - or an odd pad wear 'squealer' noise? re-reading the original post - I think an exhaust leak is a high probability. They can make a noise like you mention and cause a P0420.

make certain the tabs in the intake filter top are properly inserted in the slots at the bottom. on both my cars, the slot part is broken and I have to use zipties to secure the filter box - replacement part seems expensive. it can make a 'whooshing' noise.

why the hedgasket work? leaking externally, or was there an internal leak into the coolant? overheating events? how many and how severe? kinda wondering if the tensioner was the source of the knock sound, but not sure why it stopped. How old is the tensioner? If re-used, was it compressed very slowly? Any oil on tiop of it's cylinder? it can sound a lot like rod knock - but it may get quieter with RPMs or kinda come-and-go at different RPMs. Rod knock usually get louder and more agressive with higher RPMs. do you recall what it sounded like? misfiring in #3 dumped unburned fuel into exhaust. Cat may be damaged and blocking exhaust now. Dunno why the misfiring - valve seals? lots of guessing, sorry = hope someone else has better ideas.

seems weird, dunno that I have read of that specific set of problems unless there has been some non-OEM re-wiring of something. the A/C compressor running with defrost is the main thing you need to prevent fogging so, if there's a a/c issue - that 'may' be part of the problem. almost seems like 2 different issues. hope someone with experience with the older cars chimes in.

might be some similarity to instructions for cabin air filter change in 2nd gen ? check cars101.com . or youtube videos?

swing by a parts store and see if they can connect to it?

I think that's behind the glove box - but toward the centerline of the car. Hopefully, someone else will respond.

maybe number 17 here; http://opposedforces.com/parts/legacy/us_b11/type_45/climate/cooling_unit/illustration_1/ not sure if I have the right model, probably similar though.

you won't save any measurable amount in FWD. the frictional stuff is still turning and the xtra mass of the parts is still on-board.