I got my 2000 a4 about a week ago and decided to jack up the car and check out where the drain plug was and how to reach the filter. I was disgusted by how dirty and greasy everything was under there so I took off that cover and used a lot of degreaser and simple green to clean everything up a bit. It would be kind of annoying to be taking that thing off every time I had to change the oil, so would it be bad to just leave it off? All the other cars I've had before never had one, so I don't think there would be a problem right? Oh, and what's that wiring harness connected to the oil pan? Never seen that before.

 

leave the belly pan on, it aids in cooling.

 

If not cooling, it certainly will effect aerodynamics. I removed the belly pan from our B5 A4 V-6 QMS to see if it would improve AC performance, which it drastically did, especially when stopped. Otherwise kept it on during the other 10 months of the year when it's not so hot or going on road trips to help maintain MPG.

However it is a hassle to remove and install.

The wiring harness is for the low oil level sensor. If it comes on, it's probably already done some damage-- always check your oil at every gas fill up and keep it at the full line.

Some poor sod with a Civic Si was lucky enough that I checked his oil after doing the brakes, before test driving it. The dipstic was dry. Added 3 qts to that little 1.6 8,000 RPM VTEC grendade-waiting-to-happen. Ha, no, I didn't go over 5,000 as the car is pretty slow and the speed limit in front of our shop is 55mph on a little two lane road.