exhaust question: o2 sensor placement

[marckula]

i'm thinking about having a local muffler shop replace my shattered catalytic converter with an aftermarket replacement. i found that welding a replacement catconv. is cheaper than buying the entire exhaust assembly that contains the cat conv. so, assuming that the replacement cat. is of the correct size and supports the flow, is it ok for the muffler shop to relocate the o2 sensor bunghole anywhere upstream and close to the cat? will this hose up my OBDII system?

i have a 95 540i 6sp BTW

[JD525IA]

Could be wrong. Actually, when I changed mine, it was on top of the cat.

No?

[Blitzkrieg Bob]

Try to have the shop duplicate the stock location as close as posible.

some have them before the cat at the exhaust manifold

some have them just in front of the cat

some have them both at the mainfold and after the cat.

[Alexlind123]

On my car, it is just before the cat on the topside.

[mikell]

some have them both at the mainfold and after the cat.

The OBD II Cars have 2 O2 sensors - that did not come in until 1996.

Have them locate the sensors as close to the stock location as they can.

[marckula]

right, that looks like the stock setup, im not sure there will be a point to mount the o2 sensor on the new cat conv. if there isn't i need the best alternative. looking at the replacement part at bavauto ( http://www.bavauto.com/assets/imglib500/bmw1441.jpg ), i think putting the 02 sensor hole as close to the front of the cat (upstream) is optimal.

thanks for everyone's feedback on this.

[MBXB]

I'd mount the sensors as close to stock as possible.

http://www.realoem.com/bmw/showparts.do?model=HE53&mospid=47410&btnr=18_0222&hg=18&fg=05

[joshua43214]

Put the sensors as close to stock as possible, that way you dont have to worry about the wire not reaching. Pre obdII cars don't really care much where the sensor is , provided it is on the correct side of the cat. Not sure Post obdII sensors would care on upstream, but could cause problems on downstream.

[genphreak]

right, that looks like the stock setup, im not sure there will be a point to mount the o2 sensor on the new cat conv. if there isn't i need the best alternative. looking at the replacement part at bavauto ( http://www.bavauto.com/assets/imglib500/bmw1441.jpg ), i think putting the 02 sensor hole as close to the front of the cat (upstream) is optimal. thanks for everyone's feedback on this.Just be careful. A lot of aftermarket cats are far less than the stock unit. Stock uses twin 2" pipes to the cat- maybe even bigger on a 540. That pic (for an L6) is a good example- it looks like half the system that a 6 cyl car is fitted with stock. You only get one cat (not 2), 2-1 pipe prior and crappy pipe bends to boot. The pipes are much smaller at the cat, especially in comparison to what the car has stock... THe replacement could be good, but many aren't. It pays to be careful.