Two weeks ago I bought an 03 Baja that had bad HG. I have collected all of the parts but what type of sealant should I use for the valve covers? I am changing all of the pulleys and the belt tensioner as well as the water pump. When I removed the lower hose I noticed that someone tried to use a stop leak and there was crud in the water pump. I have a code that show that the upstream O2 sensor has a problem. Could the leaking coolant have caused this?

Your valve cover is an "o" ring it doesn't need sealant. Your head gaskets were/are leaking externally on that motor. P420 was most likely your code ,it can be caused by many things . But it is not the head gaskets.  Make sure you put japan made timing parts only . No gates kits from amazon ... they are using some parts sourced from china!  Pay more and get good oe quality parts or you will not be happy.

Edited December 17, 20169 yr by montana tom

If someone out non Subaru conditioner in the vehicle. And used a different form of stop leak, be very very mindful and keep a close on on your heater core. That and the radiator will collect most of that stop leak and cause over heating issue, lack of heat etc.

 

Not saying they're going to fail right now. But stop leak is never a fix,and only a temporary band-aid.

Definitely flow and leak check the radiator and heater core. Also consider replacing all of the several cooling system hoses, not just the 2 radiator hoses and the 2 heater hoses.