From what i've read on-line here, you should change the front (pre-cat) ones. You can buy bosch parts from an on-line retailer for about $75 a piece.

Also, there i apparently no difference in performance if you change out the two post cat (rear) ones. So I wouldn't bother with them unless you throw a code for it. They run over $90 each. Not sure why they cost more, but they do.

Try Bavarian Auto or BMA Auto parts.

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2001 BMW X5 4.4 Sport | No Mods Yet!
2001 BMW 740i Sport | 18" M-Parallel Rims | Clear Corners | AFE CAI | MAGNAFLOW EXHAUST | SOLD
1995 BMW 540i | Style 5's | Clear Corners & Sidemarkers | Smilie Headlights | SOLD