I was driving around 2 weeks ago behind a van, in heavy traffic, that was belching the most horrid, acrid smelling exhaust I had smelled in a LONG time. After around 5 minutes, my car (98 540i) began making a strange hissing noise (not unpleasant sounding) almost like an air lock type sound you hear in Star Trek or something. This continued on for about 2 or 3 minutes and then the dreaded SES light came on. The car was running fine with excellent gas mileage for a v8, and I decided to get the code read at autozone. The code came up P0430, catalytic converters. I took it to BMW, cats fine, sensors pre and post fine. Also, by the time I went to BMW, the code had erased. All they could find was 'unspecified' code. Now for the stupid question....... Could the awful exhaust of the van in front of me have fooled my sensors into this code? SES light has not come back on again.

It is possible that the car sucked in enough HC from the truck exhaust to look like a catalytic converter fault. But the intake would have to be awfully close to the exhaust and it would have to have been really loaded with unburnt hydrocarbons.

The other clue here is the hissing sound. That isn't normal and could have be an intake leak or CCV fault. The former probably would not have caused the P0430 code, but the latter certainly can.

I was driving around 2 weeks ago behind a van, in heavy traffic, that was belching the most horrid, acrid smelling exhaust I had smelled in a LONG time. After around 5 minutes, my car (98 540i) began making a strange hissing noise (not unpleasant sounding) almost like an air lock type sound you hear in Star Trek or something. This continued on for about 2 or 3 minutes and then the dreaded SES light came on. The car was running fine with excellent gas mileage for a v8, and I decided to get the code read at autozone. The code came up P0430, catalytic converters. I took it to BMW, cats fine, sensors pre and post fine. Also, by the time I went to BMW, the code had erased. All they could find was 'unspecified' code. Now for the stupid question....... Could the awful exhaust of the van in front of me have fooled my sensors into this code? SES light has not come back on again.
P.S. The catalytic converter code is PO420, not PO430.

I understand code P0420 is bank 1 and P0430 is bank 2 cat efficiency low. Both are cat codes

did you have that auto recirculated air button pushed? that might be your hissing sound.

I do use the auto recirculation feature.

Hello,
did this ever get nailed down? My dad-in-law has same code (po430) on a 94-ish 5 series.
Which side is bank 2?
thanks

Yes, believe it or not, the code finally was nailed down....Thermostat stuck OPEN!!! Anyone getting the PO430 code should look at their thermostat! Your engine will not warm to the proper temperature causing it to run closer to the start up fuel mixture. This in turn fools your o2 sensors into thinking your cats are bad, because once those cats heat up and begin lighting off, they are expecting a leaner mixture than what the engine is sending them ! Replaced thermostat for 108.00 and 2 hours on a Saturday a few weeks back, and that damned code has not yet returned :) (knock on wood) My OBD2 scanner now flashes a green light with no pendings. ( I check that thing almost every day... almost an obsession) Oh well, I'm going out to wash it in the driveway now, maybe treat the leather seats, check the tires, coolant levels, and oil etc.....Lets keep those yellow lights off !