This message appears when the rpm sensor becomes damaged or misaligned. Remove the shroud located at the front of the treadmill to gain access to this component.
The RPM sensor is located between the motor and the flywheel, there should be about 1-2 credit cards gap between the sensor and the flywheel. Large amounts of dust and debris buildup on the flywheel itself can cause the sensor to not pick up a reading, however, if the flywheel appears to be clean, the sensor should be replaced.
After replacing the sensor, you will need to run the unit through a calibration procedure. Press and hold the "0", "1", and "Start" at the same time, then release the "1" key. The display will read MAINTENANCE, then press and release the "8" key. After the "8" key has been pressed, On the newer 4000 series press the "Heart" key to perform calibration. Once the "Heart" key is pressed the calibration will perform. After the process is complete, the NO RPM error should clear itself out.