I have a carrier weather maker 8000 model #58wav070-12. It was working fine for about 3 days then stopped in middle of night. When the gas valve control switch is off I get error code 24 (secondary voltage fuse is open) when I put the switch in the on position I get error code 21 (gas heating lockout-control will not auto reset. When I disconect the gas valve I have 12.6 volts. The hot surface ignitor will not come on either.

Call a carrier dealler, tell them what you just said here, and see if they can help you out. Most likely they will have to come to your house and diagnose system themselves. Please let us know what they find.