I am in the process of upgrading the Sitecore instance from 9.0.1 to 9.0.2 [9.1 is available but we don’t have a plan to do it now]. Followed the steps in the document, 
https://dev.sitecore.net/~/media/A24AE0FB3EC947ACB8F6FCCFBA0CD8C3.ashx 
As part of upgrading the xConnect from Sitecore XP 9.0.1 to Sitecore XP 9.0 Update 2, stopped the Sitecore XConnect Search Indexer Windows service and Sitecore Marketing Automation Engine Windows service. Tried to start the services after executing the steps mentioned in the above document, but it’s keep on throwing an error. Validated multiple times, but all the steps were executed as mentioned in the document. How this can be diagnosed, it was working before executing the migration steps. Any help will be much appreciated.