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Subaru Catalytic Converter Situation

My son seems to have bought a lemon of a car last September. It’s a 2009 Subaru Forester 2.5x Premium. About a month ago he broke down and was diagnosed with bad head gaskets. Those were replaced. A week later he broke down again and it was bad catalytic converters. Both catalytic converters were replaced as well as one oxygen sensor because the original one broke when trying to remove it from the old catalytic converter and place in the new one. Four days after this fix his car broke down again (loses power, all dashboad lights flash, etc). He pulled into a Sunoco station that was right there and had them run a diagnostic. They had it on the scanner for one hour and came up with the code P0420 - bad catalytic converters. Now the catalytic converters we bought (Walker Ultra from Auto Zone) are still under warranty, and the mechanic said maybe they were just a bad parts. Before we replace them again, what are the odds that the brand new Walker CC’s were bad? Have you ever heard of this before? Could something else be the cause of the code?

It could be a few other things, and I would guess that these other issues are more likely than a catalytic converter failure because it occurred twice in a short time period. It could be exhaust leaks, intake leaks, fuel injector problems, or spark plugs. You need a mechanic to sort out which issue it is. The cheapest fix if it works is just changing the spark plugs, but throwing parts at it won’t necessarily fix the problem.

Your son did not buy a lemon, he bought a used car with problems, perhaps known to the previous owner. Why did they guy sell this Subie? You may be finding out right now. In the future, pay about $125 to get a prepurchase inspection from a mechanic you trust. That $125 is an insurance policy agains the problems you are experiencing now.


P0420 is a catalyst efficiency fault, this normally won’t cause a “break down”, it is an warning that the exhaust is not as clean as it should be.

The car seems to have a performance or drivablity problem that will need to be diagnosed. Driving with the engine operating poorly can damage the catalytic converters.

I doubt very much that the old converters were the problem in the first place.
Codes never tell you what part to replace. They only tell you from what sensor the computer received a signal that was out of the normal range that the computer expects to see.

If the mechanic replaced both cats…just from that PO420 code, he needs to be replaced.

There are many things that will trip the PO4320 code, and you need a better mechanic that knows how to interpret the code and find the true issue.


We seem to see posts about head gasket problems on older Foresters here frequently. So if it is any consolation, your son is not alone. The head gasket problem may have damaged the original cats too. Sending coolant into the cat, or a fouled-up air fuel mixture, etc. An alternate theory is the original cats were not damaged in the first place, the replacements were not necessary, and the 0420 code is caused by something else. The way it works, the engine computer monitors two o2 sensor readings simultaneously, one from before the cat, and one after. It can tell if the cat isn’t working if the two readings don’t match up in the expected way. But if one of o2 readings was bad, that could fool the computer into thinking the cat was bad, when it was really the o2 reading that was bad. Or an exhaust leak could prevent getting accurate o2 sensor readings, like a cracked exhaust manifold or bad EM gasket.

My guess, this last breakdown isn’t a cat problem, but a problem with the exhaust work recently done. There’s probably a leak that has developed somewhere. Probably not the new cats are leaking, but a leak in the joints in the exhaust pipes somewhere. Modern engines pretty much have to be air tight all the way to the exhaust tail pipe in order to run correctly. I might be inclined – if I had this problem – to temporarily remove the exhaust system completely , wear ear-plugs, warn the neighbors, and fire the engine up briefly to see if it would run ok that way. If it did you’d know the poor running was something in the exhaust system. Look for leaks and clogs, etc.

Thank you for all the input. Seems like the consensus is that the new catalytic converters are probably not the problem. My son said, when he thought it was breaking down,all the dashboard lights came on(battery, oil, check engine, etc.) and then the car choked a little, but the engine did not actually shut off this time as he pulled into the available Sunoco immediately not wanting to wait until he was on the main road. His spark plugs and wires were all changed before the head gasket job, so maybe we should have the exhaust system looked at. I wonder why the Sunoco didn’t pick up on an exhaust problem after their hour long diagnositc though. I thought the hour long diagnostic was more thorough than the quick scan. It’s really hard to know who to trust these days.

Sounds like a lot (a LOT…) of wild guessing was going on and none of the items mentioned will cause an engine to randomly shut down. The current converters are probably fine and it’s likely the head gaskets were never a problem.

Randomly shutting down would lead me towards suspecting a crank sensor or fuel pump.

I agree that this car is not a Lemon. It’s a used car whose problem I feel is made much worse by the people spinning the Faro wheel on the repairs.

The process sounds like a guy I used to work with. He’s a great guy and everyone loves him (including the people who got screwed and knew it) but he would NEVER spend one minute diagnosing anything or even trying to think it through. It was always load the parts shotgun and pull the trigger. Again. And again.