These post cat sensors will post codes, but as they dont' affect the performance of your vehicle, most people never bother replacing them.

As you've already changed the pre-cat ones, you should be in good shape and there is no real need to spend the money of the post cat ones.

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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