Have a 11 yr old Carrier 80% furnace 58RAV. Back in the fall of 2008 the furnace would go on and shutoff. Called an HVAC mechanic and they replaced a the limit switch and it worked fine for about a month. Then, every 30-45 days since the repair, we get an error code 33. I replaced the filter (it wasnt more than 30-45 days old). Problem went away for 3-045 days then happened again. Called HVAC mechanic out and he just adjusted the fan speed up a notch and replaced the filter. 45 days later it happened again. As soon as I replace the filter the error code stops. The filters are dirty, but we have gone longer than 30-45 days before the initial repair without getting the error. We will call teh HVAC mechanic again and see what the scoop is, but the last time out they didnt seem to know what was causing the error. Any ideas?