Alarm guide · Mitsubishi · Updated October 2026
Mitsubishi AL.16 / AL.20 alarm: Encoder initial communication error 1 / Encoder normal communication error 1
Mitsubishi AL.16 means the servo amplifier could not communicate with the encoder at power-up; AL.20 means communication failed while running. Common causes are a faulty or disconnected encoder cable, noise, an unstable control supply, or a faulty encoder or amplifier.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
Mitsubishi AL.16 means the servo amplifier could not communicate with the encoder at power-up; AL.20 means communication failed while running. Common causes are a faulty or disconnected encoder cable, noise, an unstable control supply, or a faulty encoder or amplifier.
Safe first checks
General guidance only. Follow the drive manual and your site's electrical safety rules. Power off, and let the DC bus discharge, before touching connectors.
- Power off and check the encoder connector at the motor and the amplifier.
- Inspect the encoder cable for damage and check the shield connection.
- Separate the encoder cable from power cables.
- Check the control circuit supply is stable.
- Try a known-good cable or swap the motor to another amplifier.
When to send it for repair
Send the motor when the alarm follows it. Send the amplifier when the alarm stays with a good motor and cable. Encoders are tested, repaired or replaced and aligned; amplifiers are tested with a motor.
See Mitsubishi servo motor repair, encoder repair and servo drive repair.
Questions
Can I clear the AL.16 / AL.20 alarm and keep running?
A reset may clear it for a while, but a feedback alarm usually comes back until the cable, connector or encoder is fixed. Running with an intermittent feedback fault can stop the axis without warning, so find the cause first.
Is it the motor, the cable or the drive?
Swap with a known-good cable first, then swap the motor to another axis if you can. If the alarm follows the motor, send the motor. If it stays on the axis with a good motor and cable, send the drive. We test both when you are not sure.
How long does repair take?
Emergency repair is done the same day where the fault and parts allow, urgent in 3 working days and standard in 7 working days, counted from when the unit reaches our bench and the quote is approved. Every unit returns with a printed test report.
Can I get an exchange unit?
If we hold a matching unit, Servo Swap ships a tested exchange unit while yours is repaired. Mark "line down" on the request form and add the part number.
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.