Whenever you remove a Subaru cylinder head you should always look at the valve guides to see if there is any evidence of them moving, particularly the exhaust valve guides. I have seen these guides moves causing misfires or even worse completely drop causing valve to piston interference. The problem seems more prevalent in the 2.5L engines but I have seen it with all 2.0L, 2.2L and 2.5L SOHC and DOHC cylinder heads. Below are some pictures so you can get an idea of what it looks like when the valve guide moves or drops. Sometime it will be just enough to knock of the valve seal and that’s it, still runs okay but starting to consume more oil than it did before.
They seem to be causing these random intermittent misfires that with OBDII becomes a problem. The misfire will be detected and the code stored. Well now (here in Oregon) your car will not pass a DEQ emissions test. Not to mention it’s annoying to have a check engine lite on. You could have other problems setting codes and wouldn’t know because you think it’s just from the intermittent misfire codes that set randomly. I have seen this problem as intermittent as once a month to a continuous misfire (valve guide dropped holding valve partially open). Check the guides for excessive clearance; if the engine overheated I have seen them shrink also. If the guide to valve clearance is okay then just knock the guide back in place and knurl it. It will never move again if you do it correctly. Over the years I have worked for shops that insist on replacing the guides, but that’s expensive and like I said they’re usually okay. Just make sure the valves are good and replace the seals.