2001 Forester Check Engine Light

Maintenance/Repairs
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1

We have been having a recurring check engine light. The code is always P0422 evac small leak. Here’s a list of repairs so far:

replaced vent valve 42084Fa120

Subaru gas cap

Subaru Dealer replaced the following parts

2 11044AA633 Gskt cylendar

4 10966AA000 Gskt Spark Plugs

2 13294AA053 Gskt Rocker

10 13271AA071 Washer Rocker

2 14035AA383 Gasket intake

1 13073AA190 Idler Cp Bel

1 13073AA142 PT340292 IDL

1 13085AA080 Idler Cp Bel

1 13033AA042 Adjuster Ay

1 21111AA007 Water Pump

1 21114AA051 Gasket Water

1 21200AA072 Thermostat

1 21236AA010 Thermostat G

2 44011AC030 Gasket

1 15208AA12A Elem CP Oil

1 11126AA000 Gasket

5 12345616 Oil 10W30Q 8.8

Next day the check engine light was back

So…found head gaskets leaking and replaced head gaskets under parts warrantee

found heads warped - sublet for repair mill and press chk heads

Next day the check engine light was back

So…replaced faulty electrical coupling.

This time the light stayed out for 3 days, but now it’s back. Help us, we love this car!

2 
I can't say all that was not needed, but much if not most of that list would have nothing at all to do with the code as reported. 

I suggest that you replace one part.   That part is the dealer or whoever is doing this work.  Find a good shop.
3

I’d start with checking the evap cannister lines going to and from charcoal cannister and fuel tank. You might have a slightly loose or cracked vac line, or shutoff valve sitting on the cannister or near it. Sounds like you did a whole lot of preventative maintenance (wow, even shaved the heads), now is the time to start troubleshooting the real problem.