M44 Giving Me a Hint? Keeps Saying 420!

[Eric]

Hi all. Got a computer code problem I have no answer for, or maybe a car that wants to drive me to Colorado.

It's a good-running, non-smoking (well at least not oil, it seems...) '98 M44 with just under 140,000 miles on it (factory longblock installed at 60,000 miles due to a spun rod bearing and no time to disassemble, clean, and reassemble the original at that point in my life). Car and engine management system have just shy of 200,000 on them.

For at least the past three years or so, the computer's been lighting the Check Engine Light about every 60-120 miles with the P0420 code (Low catalytic efficiency).

Now, I'm a patient man, and I believe in troubleshooting by substitution when possible, so I've waited for things to evolve.

The code started coming up not too long (maybe a year or two - hard to remember with the car constantly screaming for 420) after I replaced my rusted-out OEM cat. with one from BavAuto.
I changed both O2 sensors. Made no difference.
That exhaust failed 55 months into a 60 month warranty, so I got a new one (for free, no hassles - I like those people). No change.
Drove that for a year or two, then changed the O2 sensors again (Bosch). No change.
Last month I happened across an extremely low mileage aftermarket replacement cat, with two like-new O2 sensors on it. I picked it up and installed it. No change.

So, I've been through three cats and three sets of O2 sensors, with no change in the computer throwing P0420 every 60-120 miles, without fail.

Any ideas where to look next? I've gotten very good at reading and resetting my codes while I drive, but it'd be even better not to have to.

Thanks,

- Eric

[MINIz guy]

Your engine may be running rich and clogging up your cats then.

I'm not sure if you can use INPA to monitor the M44, but I would suggest looking at the fuel trims and AFR to see if your car is running rich.

Sent from my SAMSUNG-SGH-I537 using Tapatalk

[Eric]

Doesn't smell rich, and gets 29-31mpg, even with fairly "enjoyable" driving, so I don't think it's rich, but I guess it's a possibility.

And it does it right away - it doesn't take time to clog up a cat. It just throws the code.
Also, the insides of the pipes look very normal, not rich.

- Eric

[2Ti]

I tried two aftermarket cats and both threw codes not long after install. Only the OEM cat would work without throwing a code, and its been fine now for a number of years. Not sure what it is about the aftermarket cats that cause the problem. Downside is that the OEM cat is expensive, relatively speaking.

[Eric]

Hmmmmm. Yeah. I was kind of thinking that...

And, I agree, considering I paid $100 for the cat that's on there now, the OEM one is a bit pricier. I can put up with the light coming on every few miles for the price.

- Eric