Marching gathering light an Xantia 1.9 TB

Hello, I recently changed the water pump and the ditribuction strap the problem costs it to start and a yellow light is turned on is a motor gear light to which it can be. The mechanic has told me that you have to go back the belt but I do not know why when it is going well it goes well and when the march changes it will be removed if it does not pass 2000 revolutions or get into