I have a 2001 Mazda Protege 2.0 LX that’s about to go on a long road trip this week, and I want to fix the check engine light before then.
I got the code run and it’s a heated oxygen sensor issue, but it seems like my car has two of them and the online OEM replacement sensors are either “pre-catalytic” or “post-catalytic”.
I have no idea what that means, but I did manage to dig up a service manual for the 02 protege (which should be similar enough). I found a seemingly relevant page that shows the two oxygen sensors as part of the exhaust system:
“rear” is likely to be the downstream sensor, or post-cat. All cars have two (or four, for vee engines) now; the pre-cat one is used by the engine management and the post-cat one is to make sure the catalytic converter is working properly.
ETA: looked at the PDF you linked. Yes, part #5 is the downstream (post cat) sensor. #11 is the pre-cat sensor which screws into the exhaust manifold.
warm-up three-way cat. It’s the one closest to the engine so it gets up to operating temp quickly (catalysts don’t work unless hot.) Three-way cat means it cleans up HC, CO and NOx.
your car actually has two catalysts, part #3 in your diagram is the second one.
ETA:
it’s pretty simple. unplug, unscrew, screw the new one in, plug it in. it gets complicated if the old one is seized in the pipe, but for an '02 I’d think it should come out.
