The under body trays on most modern cars are used not only to protect the systems from road debris and weather but to also manage the air flow under and around the vehicle. The Audi A4 is no exception even though it does not have as many trays as most vehicles.
If you bought the car used there is a good chance that when you first get under it you will be missing one or more of the trays and/or the mounting hardware is not original and has been replaced with whatever fasteners the previous owner had lying around.
The majority of our project cars are a mishmash of weird hardware (wood screws, sheet metal screws and zip ties) and missing trays. While the type of non-original hardware is not vital as long as the trays are safely secured, missing or broken trays are a big concern. With engines packed tightly into cars these days air flow is carefully planned out for both cooling and drag co-efficiency. The trays are integrated to divert and flow air over certain components for cooling while streamlining the airflow for maximum MPG. If your trays are missing or damaged you should replace them.
You will need to remove these trays to get access to most of the work you will want to perform on the cooling systems of the car along with a host of other projects. There are two trays on the Audi A4 -- a "nose tray" that covers the front of the engine and the accessories as well as a smaller tray that covers some of the engine and the transmission.
Begin by safely jacking up and supporting your car. Please see our article on safely jacking up and supporting your A4 for additional assistance. With the car safely off the ground you can remove the trays. A great many of these car have had the engine trays removed over the years and not replaced or replaced with different hardware or fasteners. The front tray and transmission tray share three common fasteners (green arrows). To remove the front tray, remove the three common fasteners (green arrows), the eight under tray fasteners (red arrows) and the two fasteners in the wheel well (yellow arrows). Then lower the tray down and out from the engine area. Use care when lowering it. If you have had any leaks there can be fluid stored up in the tray. To remove the transmission tray remove the three shared fasteners (green arrows) and then the two fasteners at the rear (blue arrows, tray already removed). Installation is the reverse of removal.