Hello all,

I have a slight problem. Had a CEL for Cat Inefficiency. CEL comes and goes, sometimes on for 2-3 days, and then goes off, sometimes off 2-3 days. I do notice when it's on there is a slight stumble/hesitation with initial clutch releasing/light throttle. Fuel economy seems to suffer when it's on also. So, I bought a metal-core Magnaflow center-port converter, 2.25" or 2.5" ID/OD., don't remember which. Anyway, The converter was a little shorter than the OEM, so I cut the OEM out at the welds, welded in the new one back at the rear cat flange, and used a piece of stainless exhaust pipe to fill the void at the front. Replaced the o2 with a 40 dollar e-bay direct-fit. What else could be causing this? Haven't replaced the front o2, but the CEL shows downstream, not upstream. Is the placement of the converter an issue here? Doesn't make sense..

 

Aftermarket converters often aren't "efficient" enough to fix/prevent a cat efficiency error from occurring and off-brand o2 sensors are notorious for causing CELs. What's odd is that you mention the fuel economy has dropped, I'm pretty sure the rear o2 is only used for monitoring the cat while the front o2 is used for A/F adjsutments. What kind of gas mileage are you getting? If you just want the light to go away, you could try some of the tricks that guys with test pipes or high flow cats use like an o2 simulator or moving the o2 sensor out of the exhaust stream with spark plug anti-foulers.

 

What is the code? P0420?

 

Thanks for the replies. I'm going to buy 2 new NGK o2 sensors, up and downstream. I know the rear o2 only monitors cat activity, but if this car is anything like my Mazdaspeed Miata, it likely goes to a safe map when a cat issue is detected, slightly richer than the optimum map. The fuel economy may be a placebo effect just because the CEL is on.

Billman,

Don't remember. 0420 is likely it, but I believe there were staged or pending codes as well. I'll have to re-scan the car tomorrow and report back to you on that one. I'm half asleep after 12 hours at work and don't feel like messing with it tonight lol.

Goodnight all