Servo alarms look intimidating — dozens of codes across Fanuc, Lenze, Control Techniques, Siemens, and Yaskawa — but nearly all of them fall into five families with predictable field causes. Learn the families and you can diagnose most servo faults before opening the manual. Codes differ per manufacturer; the physics does not.
Family 1: Encoder / feedback faults
The drive has lost or distrusts position feedback: broken encoder cable (flexing cable tracks are the number-one culprit on moving axes), connector corrosion, EMC interference coupling into feedback lines, a failing encoder, or — on absolute encoders — a dead backup battery losing the multi-turn position. Field sequence: reseat and inspect connectors, check the cable along its whole flexing length, verify battery state, confirm feedback cable shielding is properly terminated and routed away from motor power cables. Persistent encoder alarms with a proven cable mean motor (encoder) service.
Family 2: Overcurrent / short circuit
Trips at enable or instantly at motion: suspect the motor and cable first — insulation-test both. A winding short or a chafed cable produces overcurrent that no drive parameter will fix. Trips only under aggressive moves: gains too hot, acceleration beyond the mechanics, or a mechanical jam. A drive whose output stage has genuinely failed usually trips even with the motor disconnected — a decisive test.
Family 3: Following error (position error excess)
The commanded and actual positions diverged beyond the window — the drive is saying “I was asked for more than I can deliver.” Causes in field-frequency order: mechanical binding or crash debris, undersized acceleration settings versus real inertia, load increase (new tooling, worn ways), slipping coupling between motor and load, or feedback direction/scaling misconfiguration after a repair. Following error after a motor or drive swap is almost always parameters, not hardware.
Family 4: Overload / over-temperature
The drive’s thermal model or thermistor reports sustained excess: duty cycle beyond the motor rating, ambient heat, blocked drive cooling, or mechanics that have grown stiff (dry ways, failing bearings, over-tensioned belts). Measure motor current in steady running against nameplate — a slow upward trend over months is the mechanics talking, and the alarm is the messenger.
Family 5: Bus and supply faults
DC-bus overvoltage on deceleration means regenerated energy has nowhere to go — check the brake resistor and its wiring, or extend ramps. Undervoltage points to supply sag or pre-charge issues. Fieldbus alarms (lost communication) are cabling, addressing, or a master restart — rarely the drive.
Repair, swap, or upgrade
Servo drives share the VFD aging profile: fans and DC-bus electrolytics define lifespan. One-off alarms after events are noise; recurring internal or thermal alarms on a 10–15-year-old drive argue for replacement. Like-for-like refurbished units — with parameters cloned across — restore motion the same day; note that on some platforms drive and motor are qualified pairs, so match order numbers exactly rather than “equivalent” models. Keep the parameter file of every axis backed up now, before you need it — the same rule as HMI projects.
PLC North stocks tested servo drives and amplifiers from Fanuc, Lenze, Control Techniques, and Siemens — each powered and functionally verified. Send the order code from the failed unit’s label and your alarm history; we will match stock and tell you honestly if the fault sounds like it will follow the new drive. Browse our tested servo drives and amplifiers.