Are you replacing the O2 sensors yourself? If so, read on.
The two on the exhaust manifolds usually burn-out first as they are much hotter. I replaced those two only. An engine code can be read at a parts store and they will tell you which ones need replacing. May need to look at a manual to figure out the banks they mention. Oh, and it is a Camry manual. ;)
You also have two choices of sensor styles. An OEM just plugs in, but at twice the cost. I opted for the "cut your own" type. The connector and splicers are a fabulous design. Water-proof and almost fool-proof.
Do the front exhaust first. It's the one right out in the open, on the front manifold, near the bottom. Easy to do. Though, you'll need the special O2 3/8" drive socket you can buy at a parts store.
The rear one is a royal pain! Hidden way up on the rear of the engine by the firewall. Cannot get to it from the top. Need car up on ramps, and good luck reaching it. I guess I have short arms, and could barely get to it only by feel. Much sweating and cursing seems to help, but it can be done.
Anything I can fix myself, I do. And this was one. Good luck.
