Help - BMW Forum

Ryan328is · 02-06-2009, 07:53 PM

I just bought my first BMW, a 328is. When I bought it the check engine light was on and he owner said the dealership told him it has a bad O2 sensor. The guy seemed like an honest person, he told me where the car was bought where they moved with it and that there were no accidents, carfax confirmed this 1 owner no accidents and it went excactly where he said it did so I trust that the dealership told him it was a O2 sensor. I bought a code reader to see which O2 senor went bad and it gave me the code P0420 which is "catalyst efficiency below threshold (bank 1)". I don't know if a bad oxygen sensor would give this code or a different one, and I don't know which side Bank 1 is or which oxygen sensor would most likely go out. Hopefully it is just an oxygen sensor as I could fix that for about $100. If you have any ideas please help, Thanks

marti5 · 02-07-2009, 03:49 AM

Hi, I maybe wrong but dont think so, Ive never seen a V8 328i, but if you have one, pic please, Bank1 refers to one side of a V4, V6, V8 or V12 engine.

So if yours is a straight 6 that will be the bank as you only have one.

If you have twin exhausts out the rear it maybe on about that, which is confusing as its an exhaust not a bank!!, but bank one is normally, from the front on your right.

mheidemanjr · 02-07-2009, 06:55 AM

were di the hole v8 come from?

jeewing7 · 02-07-2009, 09:17 AM

Uh... bank1 refers to cylinders 1,2 and 3. The 328 has an OBDII system and has at least two o2 sensors depending on year. The ones I've seen have O2 sensors directly below the ports where it bolts to the head.
I would simply follow the pipes to locate the first o2 sensor coming our of the 123. The O2 sensors on that model are pricey but there's no reason a universal Bosch can't be used. If you're capable of cutting and splicing wires then this is a much cheaper alternative.

Refer to the diagram here for the location.

Ryan328is · 02-07-2009, 08:38 PM

I tried to get the oxygen sensors off, but for the life of me they are not coming off. So I figured I might try resetting the code to see what happens. I let it idle for about ten minutes with no CEL, then I drove around the neighborhood, I got up to 40mph and 3k rmp and by the time I got home no CEL. The car runs flawless and the exhaust seems fine (but I can't sniff for hydrocarbons). Is there any way the CEL was a fluke or do I need to drive a lot more before it will come on? Would I pass DEQ with no CEL if it was a bad post-cat O2?

jeewing7 · 02-07-2009, 08:53 PM

The trick is in the engine. I've had good luck removing o2 sensors after letting the engine run for two minutes from a cold start.

If you've got the replacement then don't be afraid to use a vise grip or similar. If all else fails use PB blast and let it soak in for an hour.

A bad O2 sensor will cause the car to fail smog due to high hC levels. It will also cause the engine to run rich depleating fuel efficiency as well as lead to possible engine damage.

johnnyBgood · 02-07-2009, 08:56 PM

Quote:

Originally Posted by Ryan328is

I tried to get the oxygen sensors off, but for the life of me they are not coming off. So I figured I might try resetting the code to see what happens. I let it idle for about ten minutes with no CEL, then I drove around the neighborhood, I got up to 40mph and 3k rmp and by the time I got home no CEL. The car runs flawless and the exhaust seems fine (but I can't sniff for hydrocarbons). Is there any way the CEL was a fluke or do I need to drive a lot more before it will come on? Would I pass DEQ with no CEL if it was a bad post-cat O2?

Hi, if the codes were cleared, it could take a little while before you can tell.....a few days sometimes. Also on the e36 328i & is there are 4 o2 sensors, 2 pre-cat 2 post cat....I would definitley wait it out, maybe you will get lucky.....if not, there is a special tool for help removing them....link below!

BMW Parts and BMW Accessories | Bavarian Autosport

johnny,