My car has been experiencing a strange CEL pattern that is driving me nuts. It seems that for the past month every Wednesday afternoon on roughly the same section of road my Check Engine light goes on. :confused: The CEL trips and after a few days it goes out.....only to come back Wednesday afternoon in roughly the same place on I-695.

I've had the code pulled on two seperate occasions nearly minutes after it goes on and the code scanner is saying "Cat In-efficiency below threshold". Before this recent span of incidents my car has only thrown one CEL before and that was for the same code back during the winter time. I disregarded it as a fluke and saw nothing of it until late this September.

So the guys at AutoZone have told me that its either a bad O2 sensor or I've got a bad cat. However I don't think its either of the two simply because of the strange pattern that just came out from nowhere. I got a good laugh when the guy who pulled the code today suggested I stay off I-695 on Wednesday afternoons lol

But anyway my power mods consist of a Cobb CAI and an Apexi N1 catback. I appreciate any insight you guys can give me on my situation

Joe

>>I've thought about getting a high-flow cat and some headers to give the light a real reason to be on.

What's the mileage of your car. I was told by a smog tech that the 02 sensors should be replaced about every 30k miles but I haven't checked. he also mentioned checking it for continuity with a volt meter.

Just buy the hi-flo cat and mid pipe :devil:

It is most likely a bad cat. As for why it happens in a similar location each time, the ECU does not do its catalyst efficiency testing continuously. It only performs this test under certain conditions- engine warmed up, steady throttle for a sufficient period of time. In other words, highway driving. So you can have a cat that is bad or missing due to a catless exhaust, and while you are doing city-type driving the code may never come up because the ECU won't get a chance to perform its testing. As soon as you get on the highway for a while, the CEL will come on. Sounds like this is what's happening.

Mulder
That was a very, very helpful response


thank you

Joe

I was told by a smog tech that the 02 sensors should be replaced about every 30k miles:


There's a sucker born every day. Subaru O2 sensors go for 205 a pop and there are no aftermarket ones made specifically for Impreza's that I have found. They should last ~100k miles before needing to be replaced.



Mick

I got my front o2 sensor for $38 on Ebay brand new and it is aftermarket although I forgot the brand name. It has worked perfect for over 30K miles.