Beginning last summer my 2001 Forester has had a continuing problem with the check engine light, registering code PO 420. The oxygen sensors have been replaced 2X, as have the cat converter - 2X - with the cat delivered both times from the dealer. Last time my mechanic cleaned all computer leads. The light just came on again this morning (interesting that it has come on four times at exact same location on highway - assuming it's a trip mileage thing, and not some outside supernatural force, although overall driving mileage has been different between the light coming on). Any ideas? I read some places that some people just feel that the 2001 doesn't age well; but have just 92,000 miles on it, and was hoping it would be my first to reach 100K. Thanks.