Ihave an older electric forced air heating system. With the system in heat the blower will not run in the auto mode. The heat does seem to work, it will heat the house. During the airconditioning season it worked fine. During the A/C season I didhave to change a relay. I wired it IAW the diagram on the panel.

Where do I go now.

A big rule of troubleshooting is to return to the last point of repair. I repaired a broken wire at the relay I replaced this past summer.