503 error after patching windows web servers

In some cases, web application will throw the above error 503. We have seen this a couple of times post monthly patching activities.

This is how to fix it

1.      Open Application Pools -> <application name> -> right click -> advanced settings

2.      Scroll down to Process Model -> Identity -> click on the … to change the value

3.      In Application Pool Identity window,  select Built-in account and select             ApplicationPoolIdentity

             (previous setting could have been Customer account --> appadmin)