Ok, the latest:When it arrives, I'll use the borescope to confirm number and condition of cats.
Meanwhile I'm continuing to drive on premium (and combustion cleaner in the tank). P0420 hasn't returned yet. Which is weird given that the logs show the cat is doing nothing.
- the $20 ebay borescope arrived and I pulled out the oxygen sensors to have a look
- found some wet oil on the oil sump which looks like it is coming from the gasket between the oil sump and the main engine block. Looks like the garage didn't seal it up properly after they fixed the leaking head gasket. Seems to be some sort of liquid gasket - grey goo of some kind. So hopefully the oil consumption mystery is solved and can be fixed fairly easily.
- the borescope confirmed there is a second cat that doesn't have an O2 sensor downstream of it. No idea why Subaru would do that.
- I've put two videos in the Google Drive (Public Access - Google Drive)
- the resolution isn't great, but the upstream face of the cats look fine. No holes, no cracks, no sign of melting, they appear to be white in colour. There does seem to be some small particles on the substrate here and there, but I don't think that means anything?
- Last two tanks of fuel were premium with subaru combustion cleaner added via the tank. Filled up with standard (91 octane) and the P0420 code came back while driving at 100 kph on the highway about a half hour after filling up. Coincidence?
So, next step is the new aftermarket cat from the UK. I'm interested to hear if anyone sees anything on the videos that would change that plan...?
Paking89 - pdf page 775 of the fsm for my 2005 outback says the DTC will light up after 2 consecutive drive cycles that fail, and the light will go off after 3 consecutive drive cycles that pass. The prior page sets out the conditions for when a drive cycle occurs - they are pretty specific (set values for battery voltage, atmospheric pressure, speed, intake air, coolant temp etc etc) so it seems like it is hard to know when the test is performed. I'm no expert but it sounds like you've done everything and the only thing left to do is to replace the cat. If you want to be sure, have you tried doing some romraider logging? While I prefer to use OEM I can't justify it in my case - the cost of an OEM cat in Oz is almost the same as the value of my car. So, it might be worth considering non-genuine aftermarket ones? At least they are a lot cheaper in the US than anywhere else in the world!