Yo can you give me the link ? people say ebay sensors only work on the 2 middle i brought one from there and my pdc still not workingThe below opinions are based on my experience from my E39, which should be similar, as I have not had any problems with my E46.
If a sensor fails then when you engage reverse you get a continuous tone.
Sensor failure is most common followed by wiring/connectors.
If not that then possibly the module.
Hooking up the car to a laptop with diagnostic is the best way to trouble shoot. You can see a the faults on the system and run tests.
Replacement sensors from BMW are very expensive. On ebay they are cheap but some are good and some not so. I know which ones to buy on UK ebay but that won't be any use to you so others may be able to recommend.