Hi Scotty! Love your videos--- I just got a 2006 Xterra, 6 speed manual, 150k miles--- it was misfiring badly, PO300 on start-up flashing and motor shaking, and also, had a PO420 and PO430. We changed the spark plugs which had a high level of thick, white, ashy deposits on cylinders 2 and 4 with none of the six being wet, reset the SES light and it drove great.... until a few miles down the road when PO300 started flashing any time we accelerated over 35 mph though the misfires were not noticable other than the flashing light, and the light turns off on deceleration. We only drove a few minutes so the other codes did not come up, and we noticed the coolant seems to be extremely pressurized, a lot bubbles out even when opening the cap when cold. When I first got the car, the reservoir was full (the fluid looks a little murky), but the radiator was half a gallon low. Previous owner did not add fluid that I know of. The car did not overheat while running for twenty or so minutes and the pervious owner said it hadn't been. The previous owner gave us the service records and the mechanics had suggested replacing the cats 10k miles ago-- my question is, if there were a clogged cat, would it cause a blown head gasket/ the high pressure in the coolant, as that's what appears to be going on? I'd like to address the cause of the blown head gasket before putting sealer in, or does it seem like perhaps the head gasket leak is too severe to even try sealer? The car was using a quart of oil every thousand miles and we changed the pcv valve at the same time as the spark plugs as it was oily and seemed sluggish to close-- don't know if that's related or not to the head gasket. Thank you so much for your help! Your sharing your knowledge is such a kind gift to people! Sorry for this long post, hoping to have a four wheel drive car before another long Maine winter. :)

head gasket is blowing at a minimum. and engine seems worn internally too from oil useage, those are crap engines, sealer would not work or not last very long. BUT, you just got it, hope you didnt pay much, perhaps try bars head gasket sealer and then get rid of it FAST. Head jobs cost a ton on those.

Doh! I paid a thousand bucks. Going to get some Bars now. Thanks Scotty! You're the man!