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Check Engine Light "on" and "off" on 2001 Subaru Legacy GT

I bought my Subaru in 2004 with 14,000 miles on it. The check engine light began coming on soon thereafter. The Subaru dealer has replaced my catalytic converter (the so-called problem) – 3 TIMES up to 85,000 miles. The check engine has continued to come on and it goes off since then. Mostly, it’s on, but now and then, it goes off. I bought my own diagnostics kit because I got tired of taking it in every time the light came on. I’m told it’s the usual problems plus they usually have me get the oxygen sensor repaired. I’ve also been told to make sure my gas cap is tightly closed. Still the check engine light continues to come on and now and then it goes off (it’s currently “on”). I’m fed up and at a loss to understand what the “real” problem might be. Can anybody help?

If you will share the diagnostic codes (the format is similar to “P0123”) with us, we may be able to give you some asssistance. Without those codes, it is just guess-work.

It’s P0420 which reads, "Catalyst Efficiency Below Threshold (Bank 1), according to the book. Like I said, I had the catalytic converter replaced 3 times – still the light comes on. Thanks.

The gas cap advice was just plain silly.
Anyone check the sensors?

Many times (in fact I’ll say MOST times)…the problem is NOT the catalytic converter, but one of the O2 sensors. The code is telling the ECU that one of the O2 readings is not within spec. This can be caused by either the Catalytic converter not performing properly OR by a bad O2 sensor.

Find a new mechanic. A good mechanic would run further tests to determine exactly what the problem is before they just start throwing parts at the problem (in this case very expensive parts) and hope the problem will be fixed.