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2001 Subaru Outback 3 catalytic convertors PO420

We bought a 2001 Subaru Outback in Februaryt as a vehicle for getting around in snow. It has 162k miles on it. Didn’t have it a month and the check engine light came on. Took it to mechanic; they told us it was catalytic convertor going bad. It had been running fine. Then it died on me at a stop sign. It started right up. Sometimes when idling it drops to 750 then starts to bounce lower then shuts off. Plus it now occasionally shudders while driving (both at 30mph and 70mph); it feels jarring like you hit a pothole. This week we took it to the Subaru dealer. They found code PO420. They “drove vehicle while watching scan and signal was erratic”. Advised that “3 catalysts would need to be replaced”. They also said the steering pump was bad and to replace serpentine belt. I don’t know anything about cars; we’ve always had a newer model vehicle. All these repairs are going to cost about half of what we paid for it. We need guidance in deciding whether it’s worth it or not. Thanks for any advice.

It’s a common mistake to assume as your dealer did that code P0420 means that the catalytic converter must be replaced. That code can be caused by many other issues, including a bad oxygen sensor, bad coolant temp sensor, engine misfire, etc:

Do not allow anyone to talk you into replacing the cat until all the possible causes of the P0420 code listed above have been checked first.

Also, these engines are notorious for developing head gasket leaks, which can let coolant into the cylinder and in turn cause misfires which could also set a P0420 code.

The bad idle/stalling could be a symtom of a bad idle air control valve (IAC).

The shudder/jarring like you hit a pothole…that sounds like a different problem, possibly an engine misfire or a transmission problem, that a mechanic should be able to diagnose.

The car has several problems that may be related and first I would solve the stalling/possible misfire and check for a head gasket leak, both of which could be the cause of a P0420 code.

I would recommend finding a trusted local Subaru specialist instead of the dealer. The dealer will likely try to sell you unnecessary repairs and it will be extremely costly. This car needs someone who will carefully and systematically diagnose it and not just throw expensive parts on it randomly.

I don’t think that you need guidance in deciding whether it’s worth it or not. I think you need another opinion or two about your problem(s). This sounds about as much to me like a bad catalytic converter as a worn out license plate holder (i.e. not very much).

A catalytic converter that is clogged up will cause problems while driving. But the most likely things to notice won’t be erratic idle and stalling; it probably won’t just shudder (it would gradually lose power; it certainly won’t jar you on potholes.

There are an awful lot of things that can cause your problems and also either harm a converter OR provide a “false” P0420. The fancy computer diagnostics that they did are useless without an intelligent human being around to understand what is happening. At many shops, this doesn’t happen. The quickest and best route for the shop is just to throw parts onto whatever the computer said.

In any case…you probably have more error codes than just P0420. Many auto parts stores read those codes for free. It sounds, however, like you need to find a local, independent shop with a good reputation. Take the car there rather than to this dealer.

Avoid the dealership because they don’t have a clue to what your “real” problem is. Find a good independent mechanic and stick with them for all of your automotive repair needs.