Friends,
Please suggest the reliability improvement tips for Air conditioning unit (Window AC).

If you tell your problems, maybe the forum memebers will try to suggest something. Otherwise buy a book on HVAC on the book.

As in any equipment , follow the manufacturers guidelines on maintenance. If you are just starting a program start with cleaning and visual inspection. Collect data as you go so you have all the information to establish a parts stock for the units. A simple way if you are just starting out is look at your old repair orders for similar failures, then use that as a "check list" when you do a PM. Make a bullet list of items to check based on that history for a pm task, then set the task interval at a level you feel comfortable with. Put the process in place and keep an eye on the results.