P0420 & replacing the catalytic converter

I have an 03 530i that has the P0420 code that is popping and I need help with what to do. I have done some searching here on the forum and have not found the answer to my questions so I thought I would ask for help. A little history, I have had the car for 6 months and have had to do several things to it. The guy I purchased the car from listed several things that he knew were wrong. Six months later I think I’m to the end, but replacing the cat is what I’ve been avoiding, as it would be the most costly this far… The car has had multiple code over the last months, ie misfires – cam sensor, and I have worked on them one by one, down to this last one. I can give more specifics if its needed. P0420 is the only code that is coming up now. My questions are;
1. Does this mean that I need to replace the cat? I have replaced all the o2 sensors.
2. As this is bank 1 that is showing the error, can just that cat be replaced or do both need to be replaced?
3. If I need a new cat, what are some recommendations for an aftermarket cat that will work?

My goal at this point is to get it to a point that it will pass an inspection so I can sell it. Thanks for the help.

If both post-cat O2 sensors have been replaced, it's the cat. Replace both, as the other one will fail soon, too. Magnaflow does not offer a cat for your car. Unless you can find a universal fit cat, you'll be at the mercy of the dealer.

edjack, thanks for the input. bavauto has a universal cat, http://www.bavauto.com/shop.asp?HC1=...=2003&HCM=530i Is there any reason not to use one like this over one from a dealer?

i did emission testing, ase master tech with L1 and X1 catagories, so regardless of the vehicle, all cats can be tested for o2 retention and hc/co reduction prior to replacement to verify they are malfunctioning. a competant shop will be able to do these tests properly.

not sure of the mileage, but if past the 80k mark you most likely do need cats. i've had a few cases where the code popped up and the shop where i worked was simply throwing cats at the car (apparently this was fine when at a dealership under 'warranty'....) but then when a 'cash' repair came back with the exact same code after a cat was thrown at it is when i was called in to take a look.

it had new o2 sensors as well, but the culprit wasn't the cat, or the sensors, but rather the wiring from one of the rear o2 sensors rubbing against the chassis. if the cat had been properly checked with a gas analyser it would have been verified that the cat was bad. as it stood, i could not verify the previous cat condition, so to not throw the other tech under the bus (much) i stated that the harness must have been jarred loose and rubbed through the insulation causing the fault to manifest.

the cats listed in the link, however, are not 'universal'. those would be a direct replacement as it has the manifold as part of the assembly. a 'universal' unit would only have a few inches of inlet and outlet pipe to it, requiring cutting and welding to fit. it also would increase the potential for the unit to be put in backwards. (yes, they are directional, dual bed 3 way convertors, as are most all cats now).

you could check the freeze frame data for the code to see if it is consistent each time the fault pops up, but i think that most likely you will be needing to replace the cat. and, although i do not own an e39, it's quite possible the other bank will not be very far behind. (again, flying blind here without history and mileage, just my own experience in repairs)

hope this helps.


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The car has 109000. I don't know what was done prior to me getting the car and i have not had it long enough to know if there has been any loss of power or fuel efficiency as an additional clue. The code comes on consistently after it has been cleared, even after the o2 sensors. I guess at this point I'll looking to get new cat's installed.

I just stumbled upon this youtube vid about P0420 - Cat inefficiency code. Quite interesting, and could save some serious dough.

I came here trying to locate the bestlinks for catalytic converter diagnostics, removal, and repair ...

Burning lacquer thinner in my engine scares me - but then watching the guy throw the cat into the bucket of soapy water got me thinking why he didn't soak the cats with the lacquer thinner itself?

Anyway, here's what I have (so far) for the bestlinks (not much to go on):
- How to remove & replace a catalytic converter (1) & how to fix rattling cat shields (1) & P0420 codes related to cats (1) (2) (3) & cat autopsy photos (1) & what kills cats (1) & an EPA fact sheet on catalytic converter warranties (1)

Here's someone today that reported they 'hollowed out' the cats. Wow.

Does that actually accomplish anything useful?

Originally Posted by toofpik

Ive been chasing this misfire for a while and its annoying already...

My car misfires around 5k when the the check engine is on... When I erase the codes the engine goes hard all the way to the redline... When the code comes back the misfire comes back...

The code im getting is for the ICV so I cleaned it and I still got the same situation, the ICV code comes back... How can I tell when the ICV needs replacing and is the ICV even the problem? I dont have idling problems which seem to be due to a bad ICV...

Ive also hollowed out the cats, changed the plugs and did the valve cover gaskets trying to rid my car of this poltergiest... I just want some feedback before I go and spend more $$$...

Thanks in advance for any help and advice!