Rheem RGFD Furnace - no blower, flashing 68, 81
I have a 7.5 year old Rheem FGFD - 10EZCMS furnance, and the blower is no longer functioning whether the thermostat settings are on AC, Heat or just fan. In AC mode, I can hear my AC unit outside turn on, but no blower in my furance. In the Heat mode, I can hear the gas ignite, but no blower, and of course, nothing when just in the FAN ON position. The digital display on the control board flashes 81, 68, 81, 68 then solid F (which means FAN ON) for about 4 seconds then continues flashing just 81 over and over. The back panel lists code "68" under "ICM Feed-back signal" and supposedly means "no signal". Code 81 is listed under "thermistors" and means "return air sensor - out of range. Unfortunately, I do not have the furnace manual to refer to as this was installed in a new home.
Any help understanding what the problem may be would be much appreciated.