Glow Plugs on 2003 Volkswagen Jetta TDI

When replacing glow plugs, is that all that needs to be done?
What is the glow relay?

Asked by for the 2003 Volkswagen Jetta TDI
The Glow Plug Relay turns on the glow Plugs for a certain amount of time and then shuts them off so they don't melt. They literally glow inside of your cylinders to help with the cold engine start up. Why are you replacing your glow plugs? Do you have a starting problem?
My mechanic told me that a couple of plugs need to be replaced. So I am going to purchase them online and have a buddy replace them. So I was wondering if I should buy this as well. Thanks so much for your response.
Did your mechanic test resistance on your glow plugs? If they were out of spec (0.9 ohms) then you just need to replace the plugs. No need to replace the relay.