1996 bmw 318ti...engine code 420???

This is a list of the OBD II Codes.

P0420 Catalyst System Efficiency Below Threshold (Bank 1)

Your code seems ti indicate that the After CAT O2 Sensor is not working.

Your CAT could be bad, but the sensor costs in the range of $25 and the CAT is closer to $250. You pick the failure you would prefer to fix. HINT: I go with the cheapest thing first. HINT 2: There is not very much that can go wrong with a CAT, assuming you feed your car the correct kind of fuel -- unleaded.

Well except that maybe after about ten years or so the catalyst becomes poisoned and stops working. Platinum costs a fortune (~$1k/ounce or £25/g) so cat's are very expensive. OTOH the platinum can be recovered so an exchange should be a lot cheaper than ouotright purchase.

I have never seen an O2 sensor that cost $25. More like $100 and $75 if you search hard. But I do agree that the most likely failure is the sensor.

With OBDII there are specific codes (3 according to my Peake code reader) for the aftercat O2 sensor - poor response time, failed heater etc. So, it really appears that you have a bad cat - sorry. If it were just a bad aftercat O2 sensor, you are supposed to get one of the specific codes for failure of the sensor itself, instead of this poor catalyst system efficiency code. Seems like this particular code would not be useful, unless there were, in fact, the additional O2 sensor tests to verify proper O2 sensor operation and thereby validate the cat failure code.

Sorry,

Frank

Not really. I had O2 sensors go bad in both my E34 and E36, both of which also have those more specific codes and in both cases it was just the generic "out of rabnge" errors that I was getting.

I'd still do the sensors first. If it has not been changed and the car has anywhere near 100k miles it is due. If it was recently changed it ios possible the new one had an infantile failure or was contaminated by something.

Beats the heck out of replacing the cat... Those bad-boys are expensive.

Perhaps, but a new sensor costs even less, and can be replaced at home. I only suggested that it should be replaced before electing to buy a new CAT.