I have a 15 year old carrier weathermaker 8000 furnace and it has recently started making an awful thumping noise and has an error code of 12...Blower on after power up. Blower runs for 90sec, if unit is powered up during a call for heat (R-W closed). I really don't know what this means but is it fixable? and can I do it myself? Please help it's snowing I don't want to leave my furnace running.