Which parts are covered under the warranty by the general contractor?

Hello everyone, last month the external sensor of our heating system was defective. We built with GÜ and he commissioned the heating installation company. We moved in at the end of 2020. I also have a maintenance contract with the heating installation company. Now the heating installer has sent the invoice for the replacement of the external sensor. We are still within GÜ's warranty period. I would now like to know purely legally whether I can forward the invoice to GÜ and whether something like this falls under the warranty? I also thought that the maintenance contract might offer advantages in possible warranty cases. Apparently, I was mistaken.

Did you ask the GU before commissioning the heating specialists? Otherwise, he will not pay you the invoice because you did not give him the opportunity to make corrections.

The heating had simply broken down and the house was cold. So I immediately called the Heizi. If someone from the [GÜ] had reacted, days would have passed. But of course, I understand that this should have been the proper way.