Turn off the gas valve that controls the fireplace burners and wait for the insert to cool. Shut off the circuit breaker for the motor that operates the blower.
Remove the screws holding the housing on the blower motor, using a screwdriver. Slide the cover off the blower assembly and set it aside. Now you can inspect the fan blades for dust and dirt, as well as any materials wrapped around the fan shaft.
Clean the fan blades with an old cloth. Use tweezers to pull any clogged material off the fan shaft.
Pull the two wires off the motor terminals on the blower assembly. Set the multimeter to test continuity, measured in ohms, and touch the tips of the multimeter probes to the two contacts on the motor. An infinite reading or no display at all means the blower motor is likely burned out and must be replaced.
Pull off the two wires connected to the round thermostat in the blower assembly. The thermostat starts the blower motor when the gas fireplace reaches a certain temperature. Touch the multimeter probes to the thermostat contacts. A zero or infinite test with the meter means the thermostat is defective.