I've got a local 2003 Subaru Impresa 2.5L here that has a P0420.

http://www.obd-codes.com/p0420

 

What I've done.

• New Fuel Filter
• Both up and down stream oxygen sensors
• New spark plugs

I've just finished the spark plugs and the owner just left for a test drive. I'm asking if anyone has any other ideas.

 

Sounds like a contaminated catalytic converter to me. You are probably going to be replacing it real soon.

I agree with Tom. I had a 420 on my Pacifica until the cat was replaced.

Well after the spark plug change and a short test drive the CEL light stayed off so far and she is going to test drive it tomorrow on the mail route. This little Subaru has well over 300k miles (second engine) and does about 134 miles per day delivering mail out here. We'll see I'll keep you posted.

If you have a scanner that can read the mass air flow sensor and the o2 sensors make sure they are moving evenly. I've had this problem on an 05 imprezza 2.5rs and found that it was a bad mass air flow sensor.

Well that little car came by today and delivered the mail and kept heading north. I'll find out more later on today.

Update:

 

I notice the postal lady can by this evening in her other car.   Now I stop by this evening and check the error codes the P0420 is gone.   But now replaced by a P0130 code... But both oxygen sensors have been replaced already.

 

http://www.obd-codes.com/p0130