SINAMICS Fault 31905: Resolving 1FK7 DRIVE-CLiQ Encoder 12008

Problem Overview

During commissioning of a Siemens 1FK7042-2AF71-1CG0 SIMOTICS S-1FK7 synchronous servo motor on a SINAMICS drive platform using STARTER (or Startdrive), the controller raises fault F31905 with the additional fault value 12008 immediately when the user attempts to run the drive. The fault originates from Encoder 1, and the problem is reported as reproducible on this particular encoder type while other DRIVE-CLiQ-equipped motors on the same line work correctly.

Affected Components and Versions

Cross-reference the motor's MLFB (machine-readable product designation) on the rating plate before any change. The trailing 1CG0 decodes as: shaft end 1 = plain, brake F, connector C, inertia G, structural variant 0. Replacing the encoder requires the matching DRIVE-CLiQ encoder S/R variant; mixing singleturn and multiturn encoders in a configured axis raises a topology alarm that is distinct from F31905.

SINAMICS Error 31905 Decoded

F31905 belongs to the encoder-1 fault group of the SINAMICS S/G/V family. The plain-text cause changes with the firmware version and with the additional fault value (r0949 / r31321 depending on the object). Two messages share the same number — the user must always inspect the additional fault value to disambiguate.

Per the SINAMICS S120/S150 List Manual, fault values 1200x are clustered around "encoder in initialization" — meaning the drive reached power-up, polled the DRIVE-CLiQ encoder, and received a data set that did not pass the consistency check against the configured motor/encoder type.

Fault Value 12008 Meaning

The lower 16 bits of the additional fault value (r0949[0]) map to a specific cause bitmap. For 12008 decimal:

• 0x2EE8 = 12008 dec. The 0x2E00 mask indicates "encoder error during initialization"; the lower byte (0xE8) further refines the failure to "electronic nameplate could not be read / checksum error in nameplate".
• Alternative interpretation (depends on firmware branch): the 12008 value flags that the encoder identification protocol returned a length mismatch — the encoder's stored nameplate has a field length that the Control Unit's firmware does not recognize.

Root Cause Analysis

Two root cause families are credible for the 1FK7042-2AF71-1CG0 symptom:

1. Encoder electronic nameplate corruption or read error. DRIVE-CLiQ encoders carry their own electronic nameplate (encoder type, resolution, serial number, motor constants). The Control Unit reads this nameplate during the first online run. A marginal contact, an under-voltage on the 24 V DRIVE-CLiQ supply, or a stored nameplate from a motor swap (encoder swapped between motors without a STARTER "automatic configuration") will cause the CU to raise F31905 12008.
2. Manual parameter override that does not match the connected encoder. If p0400 (encoder type selection) or p0431 (commutation offset) is forced manually to a value inconsistent with the DRIVE-CLiQ encoder, the CU will reject the encoder at initialization. The symptom is consistent with the user's observation that other DRIVE-CLiQ motors on the bench work correctly — only this encoder trips the fault.

Field evidence: a motor that has been re-flashed, re-repaired, or fitted with a non-Siemens replacement encoder (some "DRIVE-CLiQ-compatible" aftermarket encoders do not implement the full namepage) is the most common source of this exact fault value.

Preconditions Before Commissioning

• STARTER installed; minimum version matches the SINAMICS firmware (e.g., STARTER V5.4 SP3 for SINAMICS V5.2). Mismatched tool/firmware versions frequently mask the real cause with a generic communication error.
• Drive powered up with all DRIVE-CLiQ cables seated; LEDs on the encoder and on the CU show steady green.
• Motor encoder port (X200 on CU310-2, X500 on the Sensor Module, or the motor-integrated DRIVE-CLiQ connector) cleaned and torqued per Siemens installation guide.
• Project offline file *.sdf (STARTER) or TIA project is current with the device version detected online.
• Read access to r0018 (firmware version), r0947[0...63] (fault code), r0949[0...63] (fault value), and p0400, p0431 (encoder configuration).

Resolution Procedure (Step-by-Step)

1. Confirm online state. Go online in STARTER, expand the drive unit, and read r0018. Note the firmware version (e.g., 5.2.3.0). Read r0947[0] = 31905 and r0949[0] = 12008 to lock in the exact fault record.
2. Automatic configuration. With the drive in "Commissioning" mode, right-click the drive in the project tree and select Drive unit > Automatic configuration (German: Automatische Konfiguration). STARTER will scan every DRIVE-CLiQ node, including the motor, and re-populate p0100, p0101, p0300, p0301, p0400, p0401, and p0431. This is the recommended first step before any manual edit.
3. Force the DRIVE-CLiQ motor re-read via p300. Set p0300 = 10000 ("read out DRIVE-CLiQ motor"). On the next parameter save / drive restart, the CU overwrites p0300, p0301, and the matched motor-equivalent data block with the values stored in the motor's electronic nameplate. The parameter auto-returns to the matched motor code (e.g., 107 for an 1FK7042) after the read.
4. Inspect p0400 and p0401 after the auto-read.
   • p0400[0] = 9999 with p0401 loaded = the encoder is being treated as a user-defined encoder. Replace with the value written by the auto-configuration (typically 9991 for the absolute DRIVE-CLiQ encoder family of 1FK7).
   • If p0400 reports "no encoder" (0) the read failed and p300 will not have re-written the data — the DRIVE-CLiQ link itself is suspect.
5. Save and restart the drive (RAM-to-ROM, then power cycle). On the next boot the CU performs a fresh encoder initialization.
6. Acknowledge F31905 and check the fault buffer again. If 12008 returns, the encoder is suspect and must be replaced as a unit (Siemens does not field-repair the encoder inside a 1FK7).

DRIVE-CLiQ Encoder Identification

To confirm the encoder being read matches the order code, inspect the live diagnostic parameters online:

If r0459.0 returns "0" or shows a coarse resolution inconsistent with the order code (e.g., a 2048-pulse incremental value where 22-bit absolute is expected), the encoder has been replaced by a non-equivalent unit — this is a frequent root cause for F31905 12008.

When to Escalate to Siemens Hotline

• Persistent F31905 12008 on a single encoder variant, with healthy behavior on other DRIVE-CLiQ motors, is a known-pattern fault that the regional hotline can match against internal Known Issue records.
• Siemens Support Request requires the full fault buffer (r0947 / r0949 / r0950 / r0968 timestamps) and the firmware versions (r0018 of every DO and CU) to attach a Service Ticket. STARTER's Online > Save all to ZIP produces a self-contained package that includes all of these.
• Warranty and Goodwill replacement of the encoder or motor is processed only via the hotline, not via the local distributor.

Verification and Commissioning Tests

1. Fault-buffer clear test. Power cycle the drive, go online, verify r0947[0] = 0. If F31905 returns with the same 12008 value, do not proceed with motion — return to root-cause analysis.
2. Encoder signal test. On the control panel, navigate to Setup > Commissioning > Encoder and run the "Encoder test" routine. Expect a green result and a measured signal amplitude within the SINAMICS tolerance window (typically 80 %–120 % of nominal).
3. Commutation test (rotating test). For a 1FK7 absolute encoder the commutation offset is taken from the electronic nameplate — no manual commutation run is required. Verify r0408[0] reports a non-zero commutation angle after the first power-up.
4. Speed- and torque-controlled test. In STARTER control panel, run a low-speed (≤ 50 rpm) open-loop command and a torque setpoint step. Inspect r0061 (actual speed) and r0080 (actual torque) for ripple and noise consistent with the motor's data sheet.
5. Long-run soak. Run the drive for at least 30 minutes at a representative duty cycle. Re-check r0947 / r2109 for any latent F31905 / A31905 entry.

Preventive Measures and Best Practices

• Always run Automatic configuration after a motor or encoder swap, before opening the drive's parameter tree for manual edits. STARTER's offline defaults from older projects are the single largest source of p0400 / p0431 mismatches.
• Keep STARTER version aligned with SINAMICS firmware. The matching matrix is published in the STARTER readme and on the Siemens support site.
• Avoid powering the motor side of the DRIVE-CLiQ chain from a 24 V supply that also feeds a high-noise load. Encoder initialization is sensitive to under-voltage and ripple; spec the supply per the SINAMICS "DRIVE-CLiQ topology" guideline (24 V ±5 %, ripple < 1 %).
• Document the encoder serial number against the motor serial number in the project tree. Siemens warranty replacement depends on this matching.
• Lock the firmware version of the drive (p7826 = 1) only after successful commissioning, to prevent the next power-up from overwriting p0300 with a re-read that re-introduces a borderline encoder.

FAQ