[cgtech]

What is this "limp mode"? Sepex yamaha Drives don't use a "reduced speed program" in the absence of a working speed sensor, in fact some claim a slight speed Increase. You can remove the sensor and check the magnet on one he motor. If you want to check the signals to & from the sensor, here's what your looking for, the red wire is 5v+, the black wire is B-, the other wire is a 5v pulse (when motor is turning). To me, Your problem sounds internal motor related.