Catalytic Converter/ Engine Misfire



I have a 2005 Impreza RS Manual Transmission, ~40,000 Miles.

During a rain storm 2 weeks ago, the car was running rough and misfiring. I brought it in and 3 of the cylinders were misfiring and they said the oil was extremely low.

They suggested a new coil and new wires for $900. My mechanic from where I lived previously had replaced the coil 6 months previous (when it was cracked).

I decided to replace the ignition wires, spark plugs, top off the oil, and reset the check engine light but leave the coil for now since it’s brand new.

A week later the car was running OK, and the check engine light came on again. Brought it in- code was catalytic converter. The dealer said there could be some unburnt fuel from misfiring and reset the codes.

Yesterday, the check engine comes on again- and it comes up as catalytic converter. Right now it is running quite rough and idling at about 200-500 rpms.

My previous dealer replaced both the catalytic converter and at least 1 O2 sensor 2-3 years ago.

Where do I go from here? Any ideas on the rough running and recurring symptoms/ codes? My last thought- I bought some really bad gasoline the other day from 7-11 (bottom of the tank).

Please help!


The computer is not saying it needs a new converter. The computer says things like "the resistance measurement from this or that part is higher or lower or inconstant etc. Then the so called mechanic just replaces this or that without bothering to see exactly what the problem is and what might have caused it.

In short, it sounds like you need to replace the mechanic. BTW Dealers are no better or worse than independent mechanics, but dealers are almost always more expensive (got yo pay for that fancy coffee somehow.)

Final note, Fast oil change places do not attract mechanics, but they sure attract some shady salesmen.

It might not be easy to do it from here, but if you give us some more data, maybe someone can help you out. Start by getting the actual codes that were read. Even if the light is not still on, you may be able to get the codes anyway (sooner the better) The code will be in the format P1234 Many auto part stores will read the codes for you for free.


The rough running engine needs to be taken care of, first. Fixing THAT could fix the catalytic converter code. The engine fuel system and air intake needs a good examination. Clean the idle air control valve and throttle plate/bore with Throttle Body Cleaner. Clean the MAF sensor with MAF cleaner.
You can bring the trouble codes here (like P0420) for further comments.


First Time the CEL came on Codes:
P0302, P0303, P0304
= Cylinders 2-4 Misfire

Second and Third Time the CEL came on CodesL
= Catalyst Efficiency Below Threshold (Bank 1)


When the engine was misfiring, was the Check Engine light flashing? And did you drive the vehicle a while with the Check Engine light flashing? If so, that’s an excellent way to damage a catalytic converter.

The reason the Check Engine light flashes under a severe misfire is to get your attention so you know continued operation of the vehicle can damage the catalytic converter.



I did have to drive it about 1/4 mile to the dealer as soon as it started misfiring (Luckily I was close and there was no other way of getting there). I believe the light was flashing and I had to start it a few times, stalled out, the cylinders were definitely misfiring.

It would make sense that the engine misfiring may have damaged the catalytic converter, but does that explain the rough running and low idiling RPMs I’m still experiencing? I don’t believe it’s misfiring at the moment and those codes aren’t coming up anymore.


On my last WRX I had what sounds to me like the problem that you’re having. Rough idle, low & inconsistent idle, misfires, check engine lights, and low oil. I had the valves adjusted & it fixed the problem.


Do you mean air intake valve? This exact problem is happening with my "03 Impreza Outback sport. Three mechanics, new plugs, new coil, throttle chamber , etc., cleaned later nothing has fixed it. It’s complicated by the fact that for me the problem is phantom–it only occurs every second or third time I start it. I have not driven it with the check engine light on, but I know it coded once for the catalytic converter after the cleaning but has since gone. I really don’t know what to do now.


A vacuum leak or misfiring could cause a low idle but what I would have a concern with would be this “oil was extremely low” part of your statement. Just how low is “extremely”?

You should be aware that cylinders and piston rings are lubricated by splash oil and continued operation with little splash is going to damage the piston rings. This in turn means engine compression problems which could cause a misfire if low enough.

Just based on what you stated about the oil being extremely low my suggestion would be to run a compression test and make 100% sure that the engine is even worth throwing parts at.
Any compression problem that exists should be diagnosed further to make sure that it’s not a tight valve lash problem that has surfaced and the rain was actually a non-factor.