Alarm guide · Yaskawa · Updated October 2026
Yaskawa A.840 alarm: encoder data error
Yaskawa A.840 is an encoder data error: the data from the encoder was abnormal. Look at the cable, grounding and noise first, then the encoder.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
The drive received encoder data it could not accept. Alarm numbers and wording can differ between series, so confirm with the manual for your unit.
Common causes
- Encoder cable damaged, loose or not shielded properly
- Noise from poor grounding or cables laid with power cables
- Connector contact problem
- Encoder fault in the motor
- Supply to the encoder out of range
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.
- Note the exact alarm code and when it appears: at power-up, at start, or under load.
- Power off and turn the shaft by hand if safe: it should turn smoothly.
- Check the motor cable, encoder cable and connectors for damage and loose contacts.
- Check what changed recently: load, speed, settings, cables or a replaced part.
When to send it for repair
If the cable is good and the alarm stays, send the motor. Encoder repair and alignment is a standard job for us.
We test every motor on our Universal Servo Motor Repair Equipment, which reads and aligns the feedback and runs the motor under load without the original drive.
See Yaskawa servo motor repair, Yaskawa encoder repair and Yaskawa servo drive repair.
Questions
Can noise really cause it?
Yes. Shield and ground faults are a common cause, especially after a cable change.
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.