From the service manual:
Abnormal condition in PF motor (X-axis) during printer operation. Displayed if the difference between motor command value and feedback from encoder is large.
1. Check error history from "Test7: Record." of selfdiagnosis function.
2. Set the number of endurance running cycles to 50 or more from “Check10: Endurance” of "Life2: PF Motor" of self-diagnosis function, and check if "X motor error" occurs.
3. Check "Encoder: PF" from "Test5: Encoder" of selfdiagnosis function.
4. PF motor assembly may be defective.
5. Main board assembly may be defective.
Stafford