[wildseamus]

Have a 02 Forester L, 150,000 miles, Auto.

Okay...

Have a 0420 code (catalyst system below threshold), in the process of resolving this problem. Tailpipe tip was sooty, my mileage took a dive, & throttle body had tons of carbon built-up. So I figured it might just be the front O2 (A/F) sensor. Cleaned throttle body, replaced fuel filter, plugs & front O2 with a Subaru OEM sensor, 0420 code gone for about 200 miles.

During the time the 0420 code was gone my code reader still gave an incomplete (Inc) monitor analysis of 02 sensor and complete (ready) analysis of; catalyst, O2 heater, misfire, fuel, comp….have known evap problem so this was Inc also. Anyhow after 0420 code returned the list of Inc is; O2 sensor, catalyst and evap. Is it possible the new O2 (A/F) sensor I installed is bad or more likely that is it truly my cat? Going to try the mechanical fix (18mm non-fouler) on rear O2 sensor in a few days to see what happens.

When I was replacing the O2 sensor I noticed coolant on the driver’s side on the steering gear boot. Suspecting the fabled head gasket leak; added the Subaru coolant conditioner yesterday (oil and coolant apperance is nomal, not milky, etc.) If it is a leak just caught it as my level only recently went down by about 1 cup since last I checked a month ago. Oil is also down maybe a 1/5 of a quart in the past 1000 miles which is a first. I remember reading somewhere that a bad head gasket can contribute to shorting the lifespan of your cat. converter as silicone in the coolant can get into the cat and wreak havoc. Anyone have any info on that….? Afraid to install new catalytic converter before I can fix coolant leak.

 

[solomondg1]

I would get a cheap cat from autozone or something and then see if it helps, if it doesn't you clean it up and return it, if one place doesn't take it take it to another.

Is the coolant on the driver side? If it is its most likely the heater core hoses that are there.

Compression test would check for internal HG leak.
Checking the coolant and oil for signs of the other also checks internal leaking.
Pop the radiator cap when the car is cold and see if any bubbles come up, this also checks for internal HG issues.
After all that the only thing left is external leaks, visually inspect the area underneath the heads for coolant/oil.
If none of these tests show anything, its safe to say its not a HG issue.