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1997 BMW 740IL catalytic converter


Car has 118,000 miles; bought in 1999 with 25,000 miles. At about 100K “check engine light” came on and was told catalytic converters needed to be replaced. No problem - had it done. Less than 2 months later “check engine light” came on again, back to mechanic, who reset computer. Drove from Ohio to Florida and light came on again. Now on third set of converters, plus new oxygen sensors and light still comes on. Local mechanic contacted BMW dealer and was told their converters won’t work on my car either. Love my car and would love some help! Anyone have any suggestions besides sell car or learn to live with the light on?

Can you please clarify the statement that BMW said that they won’t have a converter that works for you car? That seems quite odd.

A check engine light cannot tell you that your catalytic converter is bad. There is a code (P0420) that is intended for that purpose. The problem is that there are a number of things that can produce that code. So perhaps it is time to find out whether or not your issue is being diagnosed correctly.

Here is some info on the P0420:

Note that anything that makes your exhaust too “dirty” (e.g. too much unburned fuel going through; oil burning) can trigger it, as can anything that throws off the exhaust content (such as an exhaust leak). Note that in the former case, the dirty exhaust will eventually damage the new converter. Remotely, I can’t tell you what the scoop is. But I can say that I really hope that your shop has thoroughly investigated everything.

Are you sure the same P0420 catalytic converter efficiency code kept coming back?

The check engine light is just that . . . a light. To see what fault codes are present, you have to retrieve them with a scanner

It’s possible that your mechanic installed inferior aftermarket sensors that your BMW is not liking. This is a common problem. If you install new sensors again, be sure they are BMW original equipment.

Also, it’s possible the sensors are becoming contaminated from something in the exhaust, like cigroller said.

Like it was said, don’t use after market O2 sensors. Are you loosing any antifreeze? That can ruin O2 sensors along with it running too rich.

Also, how much oil do you burn? If excessive it might cause cats to fail.

Good posts above. As said, even though the code seems to imply there’s definitely a cat problem, that isn’t always (or even usually) the case. There are a number of tests required before deciding the problem is the cat. Not sure what BMW means that the new cat is not compatible with your car, but it is true that not all aftermarket cats have the same level of quality and functionality. If you like to bet on hunches, on a 1997? I’d lean toward a small exhaust system leak.

If you just want to get it fixed, take your BMW to a BMW specialist. Preferably an inde shop, but if none are available in your area, then to the dealership.

If emissions testing is not a problem put some black tape over the CEL and drive on…

I suspect installing OEM converters would total the car…(The factory parts may cost more than the car is worth)

Has anyone checked if the secondary air pump and if the pump check valve is operating?

If secondary air isn’t introduced into the catalytic converter it won’t light off and function properly.


OP said the check engine light came back on after replacing the cats

It might have come on for a different reason

There are some guys out there that just hit the “clear codes” button without bothering to check what the codes were. My scanner will let you do that, if you are so inclined.

Perhaps the guy just assumed the P0420 and/or P0430 came back

Until I hear that those specific codes came back, I’m not making any assumptions

The check engine light merely tells you there’s a problem. It doesn’t say what that problem is

I would appreciate it if OP would clearly state if the same codes came back