On the 1998-2005 Buick Regal, the engine air cleaner/filter is located underneath a diagonal brace on the driver’s side of the engine.
To check or replace the air filter:
1. Loosen the wing nut on the air duct and lift up on the two clips located on the top of the filter assembly.
2. Then disconnect the duct and re-position it while removing the side cover.
3. Pull out the filter.
4. Replace the filter if needed.
5. Be sure to install the air filter and install the cover tightly when you are finished.