A red alarm on a servo drive is the CNC equivalent of a check engine light — something is wrong, but the scale ranges from "tighten a connector" to "complete drive replacement." Replacing the drive without diagnosing first is expensive and often wrong: a surprising number of servo drive alarms are actually caused by the motor, the cable, or the power supply upstream.
Here's how to decode the alarm, isolate the fault, and replace only what actually failed.
The Golden Rule of Servo Alarms
Never assume it's the drive until you've swapped the cable. Damaged encoder cables and motor power cables are the most common cause of servo alarms — more common than actual drive failures. Water ingress in a cable, a pinched cable in a cable chain, or a loose connector can produce nearly every alarm the drive is capable of generating. The cable swap test is free and takes ten minutes. A replacement drive costs £500–1,800 and arrives next day.
Fanuc Alpha / Alpha i Series — Decoding the 7-Segment Display
Fanuc servo amplifiers (A06B-6079, A06B-6096, A06B-6080 series) show alarm codes on a 2-character 7-segment display. Here are the ones that stop production, what they actually mean, and what to do about them:
Overcurrent Alarms (The Ugly Ones)
| Code | Name | What It Really Means | First Step |
|---|---|---|---|
| 6 | L and M-Axis Overcurrent (HCLM) | Short circuit on L or M axis — motor winding, cable, or IGBT module | Disconnect motor cable at drive, power up. If alarm clears → cable or motor. If alarm persists → drive. |
| 8 | L-Axis Overcurrent (HCL) | Same as 6, but isolated to L axis | Swap L-axis cable with M-axis at drive — does alarm move? |
| 9 | M-Axis Overcurrent (HCM) | Same as above, M axis only | Swap with L-axis cable to isolate |
| A | N-Axis Overcurrent (HCN) | Same, N/spindle axis | Swap test if multi-axis drive |
| 8. | L-Axis IPM Alarm (IPML) | IPM module fault on L axis — the drive's internal power transistors have failed | This IS a drive fault. Replace drive. |
| 6. | L and M-Axis IPM Alarm (IPMLM) | IPM fault on both axes | Almost certainly a dead drive — replace. |
| 9. | M-Axis IPM Alarm (IPMM) | IPM fault M axis | Dead drive — replace. |
| A. | N-Axis IPM Alarm (IPMN) | IPM fault N axis | Dead drive — replace. |
Key differentiator: The plain codes (6, 8, 9, A) are often cable or motor. The dot variants (6., 8., 9., A.) are the drive's internal IPM module failing — that's a dead drive, no further diagnosis needed. This distinction alone saves you from chasing motor faults when the drive itself has failed.
⚡ Overcurrent in plain code ≠ dead drive. Before ordering a replacement, disconnect the motor cable at the drive and power on. If the alarm clears, measure the motor windings with a multimeter — look for a short to ground or wide phase-to-phase resistance imbalance. A motor with a shorted winding will draw overcurrent as soon as the drive enables.
DC Link & Power Supply Alarms
| Code | Name | Likely Cause | What to Do |
|---|---|---|---|
| 2 | Low Control Power Voltage (LV) | Control power supply voltage low or failed | Check the 24V control power supply — separate from the main 3-phase supply. Measure at drive terminals. |
| 3 | Low DC Link Voltage (LVDC) | DC bus voltage too low | Check the Alpha Power Supply module (separate unit) for alarms. Check DC bus fuses. |
| 5 | Over-Regenerative / DC Link Undervoltage | Excessive braking energy or weak supply | Check the external brake resistor isn't open-circuit. Check 3-phase supply isn't sagging under load. |
| 4 | Regenerative Discharge Circuit Fault (DCSW) | Brake transistor or regenerative circuit failure | Internal drive fault — replace drive. |
Thermal & Fan Alarms
| Code | Name | Meaning | Action |
|---|---|---|---|
| 1 | Internal Fan Stopped | Cooling fan failed or blocked | Clean fan and heatsink fins with compressed air. If fan doesn't spin, replace drive or fan assembly. A stalled fan will cause the drive to derate and eventually trip on overheat. |
| 7 | Dynamic Brake Circuit Failure (DBRLY) | Dynamic brake relay or transistor fault | Internal failure — replace drive. |
What You'll See on the CNC Screen
The 7-segment codes on the drive itself are the most specific. But the CNC control also displays servo alarms. Here are the ones CNC operators see:
| Control Alarm | What It Means | The Drive Code Behind It |
|---|---|---|
| SV0411 | Servo Alarm: Axis Excess Error | The axis didn't move as commanded — check for mechanical binding first, then drive alarm |
| SV0430 | Servo Alarm: Overheat | The servo motor thermal protector tripped — check motor cooling fan and motor cable |
| SV0436 | Servo Alarm: Soft Thermal (OVC) | Overcurrent protection based on rated current and time — typically motor or load issue |
| SV401 | Servo Alarm: VRDY Off | The drive's ready signal dropped — check the drive's 7-segment display for the root alarm code |
| SV5136 | FSSB: Number of Amps Is Small | Drive not detected on FSSB fibre-optic loop — check fibre cable and drive power |
💡 SV401 "VRDY Off" is almost never the final answer. It just means the servo amplifier isn't sending its ready signal. Walk over to the electrical cabinet and read the actual alarm code from the drive's 7-segment display.
Siemens Sinamics / SIMODRIVE
Siemens drives don't use 7-segment codes — they report fault numbers on the HMI or via the STARTER/SCOUT software. The most common ones that get misdiagnosed:
| Fault | Name | The Real Story |
|---|---|---|
| F30021 | Power Unit: Ground Fault | Often caused by moisture in the motor or cable — not necessarily the power unit. Megger-test the motor before replacing the drive module. |
| F30001 | Power Unit: Overcurrent | Check for shorted motor windings and damaged cable insulation before condemning the power unit. |
| F30002 | Power Unit: DC Link Overvoltage | Usually the external brake resistor has failed or the line supply voltage is too high. Not a drive fault in most cases. |
| F01600 | SI: STOP A Initiated | Safety function trip — often a loose safety relay or E-stop circuit, not the drive at all. |
| F01650 | SI: Acceptance Test Required | The safety functions need re-commissioning after a component change — routine, but requires the SI commissioning procedure. |
⚡ Sinamics Topology Fault — if you've swapped a drive module and get a topology comparison error, we have a separate complete guide: Sinamics Topology Fault Diagnosis →
Mitsubishi MDS Series
Mitsubishi MDS drives show alarms on the LED display. Common misdiagnoses:
| Code | Name | Reality Check |
|---|---|---|
| AL-12 | Memory Error | Often caused by the backup battery dying — swap the battery and re-initialise before replacing the drive |
| AL-16 | Encoder Error | Check the encoder cable before the drive. Mitsubishi encoder cables are sensitive to noise and connector corrosion |
| AL-32 | Overcurrent | Same isolation procedure as Fanuc: disconnect motor cable at drive, test |
| AL-52 | Error Excessive | The motor couldn't follow the command — check for mechanical binding first |
🔋 If the alarm is AL-12, check whether the drive's backup battery has died. The Mitsubishi MR-J2/MR-J3 battery is the first thing to try — it's a £30 fix vs a £500+ drive replacement.
Step-by-Step Fault Isolation (Works on Any Brand)
When a servo alarm appears, follow this sequence before ordering anything:
1. Read the alarm from the drive itself — not the CNC screen. The CNC display shows derived alarms; the drive's own display shows the root cause.
2. Swap the cable. Connect the suspect axis' motor cable to a different channel on the drive (if available), or temporarily swap the encoder cable with another axis. If the alarm moves with the cable, you need a new cable — not a new drive.
3. Disconnect and test. Unplug the motor cable at the drive. Power up. If the alarm clears, the problem is downstream (cable or motor). If the alarm persists, it's the drive.
4. Megger the motor. A motor with winding insulation breakdown will draw overcurrent and can damage a replacement drive. Use a 500V insulation tester — anything below 5 MΩ is a problem.
5. Check the power supply. Many "drive" alarms are actually power supply faults. On Fanuc Alpha systems, check the PSM (Power Supply Module). On Siemens, check the line module. If the DC bus voltage is low, the drives can't function regardless of their own health.
When It IS the Drive
If you've swapped cables, disconnected the motor, and the alarm persists on the drive itself — especially if it's a dot-code alarm (IPM fault) — the drive has failed. Here are the servos we stock:
Fanuc Servo Amplifier 3-Axis 0S/5S/10S — £1,177.63
Multi-axis amplifier for smaller machine tools and retrofits. View drive →
Fanuc Servo Amplifier 2-Axis 0S/5S×2 — £1,099.34
Dual-axis digital amplifier. View drive →
Mitsubishi MDS-A-SVJ-06 Servo Drive Unit — £795.95
Single-axis drive for Mitsubishi MDS series machines. View drive →
Siemens 50A LT Module 6SN1123-1AA00-0CA0 — £450
Refurbished with 12 months warranty. View drive →
We also stock encoder cables, motor cables, and drive batteries for the isolation tests described above. If your diagnosis points to the motor or cable rather than the drive, check our Servo Drives & Motors category → and Encoder Cables →.