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Z3 Won't Pass Smog

My 2000 BMW Z3 wouldn’t pass the California smog test because the catalytic converter monitor wasn’t reading. I had the front O2 sensors replaced but it’s still not reading. Mechanic is stumped because there are no codes or anything to indicate a problem with the converter or anything else. Anyone have any ideas? Thanks

It might just be the ecm’s monitor test hasn’t completed since the prior o2 sensor replacement. The computer should be telling you that though, the code should be “monitor incomplete” or something like that. Is that the code the techs are getting?

If so, do a Google search on what kind of driving is needed to make the monitor tests complete on your car. That may be all you need to do. A BMW specialist mechanic would know this, likewise a dealership shop.

You need to perform a Drive Cycle in order to get the readiness monitors to reset.

Here’s the basic drive cycle for a BMW.


Thanks guys. My mechanic is strictly a BMW shop. There were no codes. He said take it out for 60 miles or so at 55 - 60 mph. Hard to do in San Diego but I’ll try

Oh, and thanks for the link. That was helpful.

i had the same issue with my 1990 camaro z28 come to find out it was a m.a.p. sensor not sure if it would be the same issue with a bmw.

Going 60 miles at 60 mph is easy just outside San Diego. Head east into the desert or north towards LA and drive for a half hour at 60 then head back. Drive up to San Clemente on I-5 and enjoy the day.

This might be helpful.


I might add something else

Do not assume that the catalyst monitor will be complete after that freeway trip

Have somebody check that the monitor is in fact complete, before heading back to the smog station

I would hate for you to waste your time, if the car still isn’t ready

But your speed has to be relatively steady. You won’t get it to set if there’s rush hour traffic on the freeway. And it also won’t set if you’re the kind of person who constantly floors the pedal, and then slams on the brakes the next second.

I’m not implying you are that kind of person. But if you happen to be that kind of person, you may want to change your driving habits, for the purposes of getting the monitor to run to completion

I’m not making judgments, only giving advice for a specific purpose

Thanks everyone. I’ll ask about the m.a.p. sensor if the drive doesn’t fix it. I’ll have to head out I-8 I think - 60 might be too slow going up the I-5 :slight_smile: The mechanic is checking it for me so I don’t have to keep paying the smog place. He’s also trying to figure out what the issue might be since in his experience, replacing the O2 sensors usually fixes the problem. Db - no worries, I don’t speed up and then slam on the brakes - I’m an old lady - literally :slight_smile: However, trying to keep it under 60 on the freeway here is tough. The link Tester provided also mentioned that it shouldn’t go over 3000rpm during the drive cycle, so I’ll watch that as well. Thanks again everyone - I really appreciate the help!

fyi, There’s often a fuel tank level requirement too on these drive cycles. The fuel tank may have to transition across a certain fill point during the drive. For example you may have to start with a tank at least 3/4 full, and end with it less than 1/2 full. Something like that. Hopefully that’s explained in the link provided above. If not, suggest to google it before your drive, so you know what to do. Why that matters, who knows. It seems to make no sense. But if it doesn’t happen the readiness monitor won’t complete.


“He’s also trying to figure out what the issue might be since in his experience, replacing the O2 sensors usually fixes the problem”

The problem . . . by chance, did you have a P0420 “catalytic converter below efficiency” fault code, and your mechanic is speculating that replacing the front oxygen sensors might resolve that code?

Did he replace the sensors, clear the code, but now he got all the monitors to run to completion, except for the catalyst monitor?

Or did you have some kind of a lazy oxygen sensor code, or maybe even a oxygen sensor heater code?

I’m leaning towards the catalyst code, because your mechanic is perplexed

Don’t concentrate too much on keeping it under 60 on the freeway. A steady speed of 65-70 is just fine, in my experience, for the purpose of getting the monitor to run to completion

That’s the problem, evidently there were no codes whatsoever. I’m hoping the drive cycle will do it. Thanks.

And George, I’ll watch the gas level. Thanks

Good news - it finally passed! During the rain I was able to drive the slower speed for about 15 miles steady. I also did the 2min10sec idle. The mechanic had adjusted some adapters (?) too. Anyway, something worked and the cat monitor was reading. Thanks very much for all your help everyone!