Subaru and the dreaded PO420 code

Hi everyone. First post and car repair novice here.

I purchased a 2001 Subaru Forester about 4 years ago. Within 48 hours of buying it from a dealer, the check engine light came on showing the PO420 code. The car was running fine without any problems and it was suggested to us by 2 independent mechanics that Subaru is notorious for throwing this code, so we didn’t investigate it any farther besides doing a fuel system clean/flush that was offered to us by the dealer.

Lately, the car has started sort of shuddering during acceleration so we figure it’s time to look into this more, but just trying to figure out where to start. From what I understand, this model has 2 catalytic converters and 2-3 O2 sensors. I’ve been reading as much as I can on the topic and watching videos trying to figure out our options and whether we should try replacing the O2 sensors first or just go for replacing the whole shebang. I know we cannot afford the pricey repair of it all right now so the car will have to sit for quite some time before we can do that; however, if we can figure out how to do it ourselves, we could get it done much sooner.

My questions are:
What is the likelihood that this could still be an O2 sensor issue considering the shuddering when accelerating?

What other signs do I look for besides rattling in the cats to know they might be bad?

Is it possible to find an after market part that is worthwhile? I hear back and forth on whether Subaru parts are necessary for this. A $150-300 price tag sounds a lot better than the $600-800 that I’ve seen so far.

Considering that we are total novices, how challenging would this repair be to do ourselves? I’ve been looking at no-weld systems that bolt in instead.

Thanks for any and all info!

So you’ve been driving with the P0420 code for 4 years?
Don’t blindly throw parts at it.
Get the codes read again and post them here.
Shuddering sounds like there’s misfiring.
How many miles on the plugs?
Are they NGK or Denso brand? Those originals are the only ones you should be using.
Is there a changeable fuel filter?
Have you been doing the maintenance in the Owner’s Manual?

If your check engine light is still on, get the codes read again. You probably have new codes and one or more of them is the actual cause of the shuddering. Post the actual codes here.

You have three O2 sensors, two in front of the cat and one behind it. Typically the one in the rear is the one that causes the P0420 code, but that is only about 40% of the time. First get the code read again before doing anything else. If you don’t, you will likely waste a lot of money.

We have been driving with it, unfortunately. We were too nervous to invest the $ since we had a friend who spent thousands and still has the code. I tried talking with several different mechanics about that specific code and none of them wanted to do anything but replace the cat.

The code isn’t always there, it comes on and off. Usually clicks on if we have been driving for at least 30 minutes.

I would have to check my books and see about the plugs, I am not sure how long it has been since they and the fuel filter were changed. We have been doing all recommended maintenance.

Just had the code read again this morning. Only the PO420 code showed up.

Start with new spark plugs, and go from there.

I wouldn’t worry too much about the fuel filter, but how long has it been since the air filter was changed? It wouldn’t hurt to do the plugs and the air filter, then the rear O2 sensor.

Took it in again today. Mechanic says it’s likely the spark plugs and to look for valve leaks so we’re going to start there. Prob going to replace that rear O2 sensor as well. Thanks for the input everyone!

There’s a thread here from a week or two ago about a dual exhaust/dual cat Ford Mustang with the 0420 problem too. It’s a tricky thing to diagnose. You might want to read through the ideas in that thread.

The shuddering during accel could be another symptom of a bad cat, if the cat is partially plugged up, and it shows up when the engine is trying to move exhaust gas out through the cat and the tailpipe as fast as possible, which is during hard accelerations. B/c you have that symptom, ask your shop if they have the ability to test the exhaust back pressure. Even checking the intake manifold vacuum reading might provide a clue.

Thanks George. I will look into that as well.

My husband said that it has been the worth when trying to start driving from a complete stop and today backfired a little bit.