Bmw z3 catalytic converter problems

Hi I wonder if anyone can think of why i am on my 3rd catalytic converter in the last 2 years? I have a 1998 bmw z3. could it be because i am not using premium fuel? or could there be something wrong with the engine? it runs great.



The first thing we need to know is - why are you replacing the catalysts?

Hopefully, they are not being replaced just because the check engine light illuminates and the computer is reporting low catalyst efficiency. That code rarely indicates an actual catalyst problem.

The only things I know that destroy a catalyst are corrosion, heat, and contamination.

Corrosion comes from salt on the road and age. Can also come from buying fuel from crooked vendors who are mixing waste industrial solvents in with their gasoline. A bit of halogenated cleaning solvent in the mix creates hot acid vapor when it burns. That corrodes everything.

Is the catalyst getting red hot and melting down? If so, then you have a misfire that is letting excessive unburned fuel get to the catalyst where it burns. The catalyst is made to run hot, but not red hot. A misfire should set a check engine code, but sometimes does not. Pay close attention for any engine roughness, especially at high load and RPM, like when you are climbing a hill.

Contamination - we are back to wondering where you buy your fuel, or whether you are using any additives in your fuel or oil. Does the car consume oil?

Using regular fuel should not bother a cat. A '98 Z3 should be tolerant of regular fuel anyway - it just retards its timing a bit and runs fine. Newer BMWs are not nearly so forgiving.

There’s two ways catalytic converters are damaged. Either they’re poisoned from a foriegn substance in the exhaust stream such as coolant, or a silicone sealant that isn’t sensor safe used on the engine. Or they become damaged from thermal shock. This happens when raw fuel from the engine enters the catalytic converter causing it to overheat. This results in the substrate in the catalytic converter to either melt or break apart. So depending on how the catalytic converters are being damage will dictate where to look for the cause.


Just to throw this out here…not trying to blame anyone, but i have seen many cars with cheap aftermarket catalytic converters installed…These universal cats only last a year or so. A quality cat will cost you between $450 aftermarket or near $1000 from the dealer.

hi, thanks for your reply, initially just after I bought the car it had a sudden loss of power and mechanic said it was the cat. then for two years in a row it failed its MOT (annual check in UK) due to excessive emissions. it may be that the parts they have used are lousy. I have never mixed fuel with anything and buy from larger gas stations like shell so there should not be any problem in that dept.

Is your car a “preminum required” car? change converter manufacture (unless you get warranty for parts and labor).

Perhaps if you could post what emissions it ffailed for and the readings that data would give us a clue.

Maybe the problem is not the converter but is a botched diagnosis; not uncommon for converters and O2 sensors.

I find it hard to believe that the converter was responsible for a sudden loss of power. That sounds more like a fuel pump or ignition failing.

As to failing the MOT the reasons why really need to be known along with any codes that may exist and a general rundown of the maintenance history on this car as to mileage, how often spark plugs are replaced, etc, etc.