Siemens Fault 0a7991 Best !free! May 2026

Mastering Siemens Fault 0A7991: The Best Troubleshooting Guide for SINAMICS Drives

Date: October 2023
Product Scope: SINAMICS G120, G120C, G120D, S120 (CU2xx-2 series)
Keyword Focus: siemens fault 0a7991 best

✅ Step 2 – Check grounding & shielding (most effective fix)

Verify motor cables are symmetrically shielded and shield clamps are 360° bonded.
Ensure low-impedance PE connection between drive, motor, and cabinet.
Separate control cables (analog/digital I/O, encoder, fieldbus) from power cables by at least 20 cm.
Check if the fault appears when a specific load is applied (e.g., high current). If yes → likely EMI.

✅ Step 4 – Update firmware

Use SINAMICS Startdrive (TIA Portal) or STARTER software to check for firmware updates.
Update CU and power unit firmware to the same compatible version.

Phase 1: Reset and Prep

Acknowledge Fault: Press the "FN" + "OK" (BOP-2) or send fault reset via control word.
Set Basic Parameters:
- p0010 = 1 (Commissioning mode)
- p0304[0] = Rated motor voltage (e.g., 400V)
- p0305[0] = Rated motor current (Amps)
- p0307[0] = Rated motor power (kW)
- p0308[0] = Rated motor power factor (cos φ)
- p0310[0] = Rated motor frequency (Hz)
- p0311[0] = Rated motor speed (RPM)
Exit Quick Commissioning: Set p3900 = 3 (End quick commissioning with calculation).

The Top 5 Causes (The "Best" Diagnostic Checklist)

To solve this fault efficiently, you must audit these five states in order. siemens fault 0a7991 best

The Core Meaning

The drive is telling you: “I cannot measure the motor because I am not in the correct operational condition (OFF1 or OFF2 active, or I am still processing another command).” Verify motor cables are symmetrically shielded and shield

Quick Reference

Fault Code: 0A7991
Fault Description: "Encoder 1: Zero pulse lost" (or similar variation depending on drive series).
Severity: Drive typically trips and stops.
Core Issue: The drive cannot detect the reference (zero) marker on the encoder cable during rotation.