Code 231120: <location>Encoder 1: Power supply voltage fault
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical <location>Encoder 1: Power supply voltage fault alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.
Official Troubleshooting Steps
Message value: Fault cause: %1 bin Drive object: All objects Reaction: A_INFEED: NONE SERVO: ENCODER (IASC/DCBRAKE, NONE) VECTOR: ENCODER (IASC/DCBRAKE, NONE) Acknowledge: PULSE INHIBIT Cause: Encoder 1 power supply voltage fault. Note: If the encoder cables 6FX2002-2EQ00-.... and 6FX 2002-2CH00-.... are interchanged, this can result in the encoder being destroyed because the pi ns of the operating voltage are reversed. Fault value (r0949, interpret binary): Bit 0: Undervoltage condition on the sense line. Bit 1: Overcurrent conditi on for the encoder power supply. See also: p0491 (Motor encoder fault response ENCODER) Remedy: For fault value, bit 0 = 1: - correct encoder cable connected? - check the plug connections of the encoder cable. - SMC30: Check the parameterization (p0404.22). For fault value, bit 1 = 1: - correct encoder cable connected? - replace the encoder or encoder cable.
Source: SINUMERIK 840D sl / 840Di sl / SINAMICS S120 Diagnostics Manual (DAsl, 03/2009)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Siemens SINUMERIK 840D CNC Alarms
Siemens controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 231120 on the SINUMERIK 840D CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Siemens's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4014 Siemens SINUMERIK 840D CNC codes.
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