Resolving Sinamics G120C F1611 Fault Code 1951 on FW 4.7

Problem Overview

The SINAMICS G120C compact inverter family (frames FSA through FSC) running firmware V4.7 intermittently reports fault F01611 with the extended error code 1951 (hex 0x079F). The fault belongs to the Safety Integrated (SI) fault group and surfaces as "SI CU: Defect in a monitoring channel" in STARTER, Startdrive, and the operator panel (BOP-2 / IOP-2). The 1951 sub-code annotates the underlying message as "Module temperature not plausible", meaning that the difference between the Control Unit internal sensor and the Power Module sensor has exceeded the safety firmware's plausibility window.

Unlike the majority of SINAMICS faults, a standard acknowledge (OFF1/ON sequence, p3981, or digital input via the control word bit 7 pulse) does not clear F01611. The drive demands a full deselect and re-select of the active Safety Integrated function before the safety pulse inhibitor re-arms. In plants without permanent engineering access, this means a STARTER/Startdrive operator must be on site.

Affected Products and Firmware

Siemens has confirmed that the V4.6 baseline does not exhibit this fault under equivalent load profiles. V4.7 HF releases supersede the original V4.7 build; verify the exact Service Pack level on the type plate of the CU or via parameter r0018 (firmware version).

Root Cause Analysis

Siemens engineering traced the F01611 / 1951 event to a combined EMC and software timing condition inside the crosswise-compared safety monitoring channels of the SINAMICS G120C. Three contributing factors are reproducible on the bench:

1. EMC coupling into the temperature sense path. Fast switching edges from the IGBT gate driver and externally routed signal cables inject common-mode noise onto the NTC/PTC sense lines routed on the Power Module. The differential plausibility check in firmware V4.7 evaluates sensor samples within a tighter time window than V4.6, so a single noisy sample window trips the comparator.
2. Asymmetric watchdog sampling. The dual-channel safety monitor on the Control Unit samples the temperature register at staggered intervals. Under load with marginal EMC margin, the two channels occasionally latch different timestamps, invalidating the cross-check.
3. Ambient/power module thermal gradient. Repeated high-load ramps with short cool-down periods widen the Control-Unit-versus-Power-Module temperature delta. The plausibility window in V4.7 was tightened to comply with a revised internal safety case, which exposed marginal installations previously tolerated by V4.6.

The official Siemens statement characterizes the root cause as "a combination of EMC and software timing" pending the V4.7 hot-fix release. Engineering change requests for V4.7 SPx widen the plausibility tolerance and add a debounce filter on the raw temperature samples.

Diagnostic Parameters and Buffer Reading

Before clearing the fault, capture the diagnostic buffer so the failure mode can be analyzed. All parameter numbers below are SINAMICS standard indices (refer to the SINAMICS G120C List Manual).

PROFIdrive Status Word 1 (r0968) Bit Map

After F01611 trips, parameter r0968 (Status word 1, PROFIdrive profile) reports the drive state machine position. The bit map is the same as the SINAMICS G120, G120C, G120D, G120P, G130, G150, S110, S120, S150 families:

PROFIdrive State Diagram (SINAMICS G120C)

Clearing the Fault On-Site

Because the safety channel has been latched, the standard p3981 = 1 (acknowledge drive faults) only resets the non-safety portion. Follow this sequence:

1. Open STARTER (V5.x) or Startdrive (V15.1 or higher in TIA Portal).
2. Go online with the affected Control Unit.
3. Navigate to Drive → Safety Integrated → Functions.
4. Read the active safety function status. Confirm that SS1, STO, SLS, SDI, or SBC is currently selected.
5. Deselect the active safety function by writing a zero to the PROFIsafe control word or by removing the safety enable on the configured terminal/F-DI.
6. Wait for the safety pulse inhibitor to drop (typically 5-20 ms). Observe r9772 (Control Unit safety status word 1) bit 0 transition from 1 to 0.
7. Issue the standard acknowledge: set p3981 = 1 or send control word bit 7 = 0→1 rising edge on the process data.
8. Re-select the original safety function. Verify r9772 bit 0 returns to 1.
9. Read r0945[0] to confirm F01611 has cleared. The buffer retains the historical entry.

Workaround A — Firmware Downgrade to V4.6

The interim Siemens recommendation is to downgrade the CU240E-2 firmware from V4.7 to the latest V4.6 Service Pack. This restores the wider thermal plausibility window and the older watchdog timing.

Prerequisites

• STARTER (V5.4 SP3 or later) or Startdrive (V13 SP1 or later).
• The official V4.6 SPx firmware package, available from the Siemens support entry 67385235.
• Backup of the current parameter set (Drive → Commissioning → Save parameters to PC).
• Documented list of all safety-relevant parameters (p9601, p9602, p9610, p9611, p9620, p9650, p9651, p9652, p9761 through p9763, p10201 through p10215).

Step-by-Step Procedure

1. Power up the drive in a safe state. STO must be selected before any firmware update.
2. In STARTER/Startdrive, connect online and execute Target system → Load firmware (or right-click the drive → Load to file system / Load FW update).
3. Select the downloaded V4.6 SPx file. Confirm the cartridge or CF card image.
4. Execute the download. The Control Unit will reboot automatically (LED pattern: RDY green flashing → RDY green solid, around 60-90 s).
5. After reboot, the drive reports the original parameter set. STARTER may show configuration inconsistent; perform Load to PG and reconcile.
6. Verify r0018 reports the V4.6.x build.

Safety Acceptance Test After Downgrade

Per SINAMICS G120C Safety Integrated Function Manual, a firmware change to the Control Unit requires a renewed acceptance test. The test must verify all configured safety functions and must be documented with signatures.

Workaround B — EMC Mitigation Without Downgrade

If a downgrade is not feasible (for example, when the V4.7 baseline is required for a third-party motor type), apply EMC hardening to the existing V4.7 firmware.

1. Shielded cable for the motor feeder. Use a symmetric, four-core, shielded motor cable (e.g., Siemens PROTOFLEX-EMV) with 360° EMC glands at both ends. Connect the shield to the PE bar inside the drive cabinet and at the motor junction box.
2. Separate signal and power routing. Keep the encoder cable and any safety input wiring at least 200 mm from the motor cables. Cross at 90° if intersection is unavoidable.
3. Install a line filter. Siemens recommends a Schaffner FN3270 or equivalent foot-mount EMC filter at the line input. Refer to the G120C manual's EMC installation guideline.
4. Ground the CU240E-2 PE terminal with a short, low-inductance strap to the cabinet backplane. The PE conductor cross-section must be at least equal to the line conductor cross-section (per IEC 61800-5-1).
5. Check the DC link grounding. For PM240-2 frames FSA-FSC, verify the EMC plate is bonded to the cabinet backplane across its full surface, not via a single pigtail.
6. Reduce switching frequency if the load profile allows: p1800 from 8 kHz to 4 kHz reduces dV/dt noise on the motor feeder at the cost of higher motor losses.
7. Add a dv/dt reactor between the drive and motor for cables longer than 25 m. This dampens reflection-induced ringing that couples back through the sense lines.

After each EMC modification, run a thermal-load test that mirrors the original failure scenario: rapid accel/decel cycles with short cool-down periods. Monitor r0035[0] and r0035[1] and log the delta. A delta stable under 5 K over the test window is a positive indicator.

Reporting the Fault to Siemens

Open a Technical Support Request through the official Siemens Industry Online Support portal at SIOS Support Request. Include:

• MLFB of the Control Unit (read from r0010).
• Firmware version (r0018) and any installed Hotfix identifier.
• Drive trace exports showing the 1951 event from STARTER's trace function (signals r0035, r0036, r0945, r9772).
• Photograph of the type plate on the Power Module.
• One-line diagram excerpt of the cabinet wiring, including the EMC filter and PE bonding.
• Description of the load profile and any environmental conditions at the time of fault.

Reference the internal case number once Siemens assigns one. If you are operating a certified safety chain (e.g., SIL 2 / PL d under IEC 62061 / ISO 13849-1), flag the request as safety-relevant so it is routed to the safety engineering desk.

Verification Matrix

Long-Term Outlook

Siemens engineering is delivering a corrective release within the V4.7 Hotfix line. The fix enlarges the temperature plausibility tolerance and applies a software debounce on the raw temperature ADC samples taken before the crosswise comparison. Once a customer receives the new build, repeat the acceptance test and update the safety file with the new firmware CRC. Rollback to V4.6 should be treated as a temporary measure, not a permanent engineering solution.

FAQ