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Subaru code catalytic converter,

High all, my 96 Subaru 2.5 Outback has thrown a catalytic converter code. I replace the converter and it is still showing the same code. Also, the car is running bad. It has a low speed miss and a high speed miss and jerks when it shifts. It was doing this before the replacement and after. Could it be the oxygen sensor? I am going to put new spark plugs in today just in case but I don’t want to throw expense parts at it until I can indentify for sure what it could be. Give me some ideas of what to check.


The engine is misfiring. That, and excess unburned fuel fouled the old catalytic converter, and will foul the new one, unless you fix the spark and fuel issues.
Changing the spark plugs and wires is a start. Also, change the distributor cap and rotor, and the fuel filter, and air filter.
When all the routine maintenance is performed, you can begin troubleshooting. A repair manual, and a digital multimeter, a fuel pressure test gauge, and a vacuum test gauge will help you tremendously.

Sorry to hear you’ve already thrown an expensive part at the problem. I agree the miss will stress the cat converter, but the old one was not necessarily ruined. There is no code that says directly the converter is bad. You learned that the hard way. The code says there’s a problem in the system. It could be an O2 sensor. It could also be an exhaust leak or even a vacuum leak, which would also cause a miss. The thing to do is look at the signals coming out of the O2 sensors with a scanner or a scope. Unfortunately there are not enough auto techs with troubleshooting talent.

You need to stop buying expensive parts because of computer codes. It would be a lot cheaper to just have a professional mechanic fix the real problem…

No codes that reflect that the catalytic converter is bad?

P0420=Catalyst System Efficiency below Threshold (Bank 1)
P0421=Warm Up Catalyst Effeiciancy Below Threshold (Bank 1)
P0422=Main Catalyst Efficiency Below Threshold (Bank 1)
P0423=Heated Catalyst Efficiency Below Threshold (Bank 1)
P0424=Heated Catalyst Temperature Below Threshold (Bank 1)
P0430=Catalyst System Efficiency Below Threshold (Bank 2)
P0431=Warm Up Catalyst Efficiency Below Threshold (Bank 2)
P0432=Main Catalyst Efficiency Below Threshold (Bank 2)
P0433=Heated Catalyst Efficiency Below Threshold (Bank 2)
P0434=Heated Catalyst Temperature Below Threshold (Bank 2)

Everytime I pulled one of these codes, it turned out to be a bad catalytic converter.


Well, a professional mechanic did the diagnostic on this and this is what he said to do! I saved a ton replacing the part myself so I don’t see the savings you are talking about. I came here to get an idea of where to look next since the “professional” sucked and cost me money. I had a suspicion from the beginning that it wasn’t the cat, or at least not the cat that was causing the miss. What do you know,I was right and the “pro” was wrong, and I build furniture for a living! In this economy I have to do all I can to save money, even with the new cat I am still ahead of the game. Now, I will try new plugs, wires, filters since we have neglected that for some time now, and will look for vacuum leaks and exhaust leaks as other have suggested. Thanks for the tips

So what codes did the professional mechanic give you and are you absolutely positive the mechanic did not use words like “could, maybe, might be, can be, etc, etc” in there along a disclaimer that the problem could be caused by something else?

Sorry, but don’t blame the mechanic because you did not want to pay him to do the job.
It sounds as if your car has been neglected because you were obviously driving it in a poor running condition before, did not want to pay someone to do the job, and from the tone of your post I gather you flat don’t want to spend money on the car.

With a poor running car step one should be removing the spark plugs and running a compression test. Given the fact it’s a Subaru which can be prone to head gasket problems it’s even more critical to do this. Follow this up with inspection for vacuum leaks, plug wire inspection, and any one of a dozen other things.

All of these diagnostic steps take time, which means money. So, are you willing to pay the mechanic for any and all diagnostic steps required to pinpoint the problem?

You build furniture for a living. So does this mean if that mechanic asked you how to repair the finish on a tabletop or an armchair leg and it doesn’t come out right that it’s your fault?

First off I apologize for ranting but I felt I was on the defensive from the previous post telling me to go to a pro. Secondly, the mechanic did tell me without a doubt it was the cat and he never used the word could, maybe, etc… He was paid for the scan. Thirdly, the car has not been neglected. It has been a while since I replaced plugs and wires but it was running great. The problem came on suddenly, that is why I doubted it was the cat. As far as spending money, you are right. As I said previously money is tight around here and I am just trying to save money. Why would this forum be here if everyone took their car to a pro?
Now down to business, I have a compression gauge and I do like your thought there. I will do that this week. That is the kind of advice I’m looking for, not attacks on the care I give my vehicles.
As far as me helping someone with furniture, if they did exactly as I told them and it didn’t work I would go check out what they did and if it was my fault I would do it for them at no charge.

“No codes that reflect that the catalytic converter is bad?”

That’s not what I said. And I did not say it couldn’t be the converter. I suggested further testing.

Tester, It looks as though you have fallen for the myth that DTCs (Diagnostic Trouble Codes) equal defective parts. I don’t believe you that “every time these codes come up, it’s because the catalytic converter is bad”.
There are far and away too many “professional” mechanic who follow that same myth. They don’t fix the cause, they remove the symptom, and the motorists suffer monetarily from it.

The reason I mention a compression test is is that it only takes one cylinder being abnormally down a bit to create all kinds of grief.
I was a mechanic (aircraft) when I first started working for car dealers and both my aircraft instructors and the first dealer I went to work for (who was very reputable and extremely thorough in their procedures) trained me well; at least in my opinion.

You always weed out a mechanical fault first by using the compression tester.
As an example of “you never know”, we had a Subaru into the shop one time in which the cylinder heads were trashed so bad that both of them were non-repairable and had to be replaced; and this car only had 7k miles on it.
Over the years, I have seen countless cars that had every part in the book thrown at them when the problem all along was a weak cylinder or a worn, tired engine.

Since you can do some of your own repairs, I would suggest adding a vacuum gauge to the toolbox. This is one of the most valuable, and cheap, tools you can own. It only takes a few seconds to connect and much can be learned about an engine just from watching that needle. It can tell you if there’s a head gasket or low compression problem instantly. (although it won’t be cylinder specific)
There’s a reason why piston engine airplanes all have them.

I’m not defending the mechanic and it could be that he was simply wrong. it’s also possible the problem could be the O2 sensors or an engine performance problem causing this. O2 sensors are somewhat delicate and they will not take much banging around or extreme force that may be used to remove and install them.
A converter, just like an engine computer, can only control things up to a point and if the engine is running badly for whatever reason you may very well get some codes set related to the converter.

Anyhoo, hope some of that helps and if you run a compression test you might post the numbers back. Depending, and if everything is in order, you should have readings of (roughly) 170-190 PSI on all cylinders although 190 might be a stretch on an aged car.

The condition and appearance of your old spark plugs can tell you a lot about how each cylinder was doing. Compare your spark plugs to these:
The vacuum gauge can provide a lot of information about your engine, and it’s very easy to connect. This very informative site will show you what to look for. Click on each green “Scenario”:
The compression test gauge can tell you about the physical condition of the engine which would otherwise require a tear-down to determine.

Hi, I had the same problem with 1999 outback wagon 2.5 It was the KNOCK SENSOR. When knock sensor is bad it allows gas to build up and instead of burning it explodes. It can cause catalytic converter problems or worse damage. Easy to replace on most vehicles. You Tube has videos of how to replace knock sensor in subaru engines. Hopes this helps.

Thanks for bringing up the knock sensor as a suspect. When you say you, “had the same problem”, did you mean that you had the same DTC P0420? If so, there should have, also, been a DTC (Diagnostic Trouble Code) for the knock sensor. Was there?
The engine computer runs the engine on the edge of knocking. If the knock sensor/circuit goes out, the engine computer may allow ignition timing to advance to where knock (ping) occurs. This can allow unburned fuel to go into the catalytic converter and burn, which will overheat and (perhaps) melt the catalytic converter, internally.

Perhaps, you could bring us up to date on the latest development with your Subie? 'Twould be nice.

My car had no power-nearly stalling, fuel mileage dropped, I smelled raw gas, the car jerked when trying to quickly accelerate. My Mechanic did find the problem thru diagnostic code, but I don’t know what it was. My Subaru runs like a new one after replacing the knock sensor! I also had a code for low catayst, probably need cat converter. Have a good day.

A catalytic converter rarely goes bad all by itself. It’s damage is done by things, such as an engine running too rich, misfiring, out of time, in the ENGINE, not in the catalytic converter itself. So yeah, the knock sensor could have KNOCKED out the catalytic converter.
You, too, have a great day.

This is a fantastic post…I totally agree with you ok4450. Although it’s taken for granted with these modern cars to just plug them into the diagnostic brain-box for answers to problems, at the basic level the automobile is still a mechanical device and mechanical problems should be weeded out first. I agree that a vacuum gauge should be in everybody’s toolbox…very useful tool.