Siemens Fault 0a7991

While the exact wording may vary slightly depending on the CPU family (S7-300, S7-400, S7-1200, S7-1500), the fault typically translates to:

"I/O access error when updating the process image input table. The module is either defective, incorrectly parameterized, or not physically present."

In some Siemens documentation, 0A7991 is linked to a "Module failure" or "Channel error" on a specific slot of a remote I/O rack (ET 200SP, ET 200M, or ET 200S).

| Step | Action | |------|--------| | 1 | Clear mechanical obstruction. | | 2 | Increase torque limit (temporarily for testing). | | 3 | Check brake control logic & power supply. | | 4 | Perform encoder test (p2500, p2504). | | 5 | Recalculate motor parameters (p304, p305, p311). | | 6 | Disable "stall detection" temporarily (p2178) for diagnosis only. |

Note: Do not disable stall protection permanently unless advised by Siemens support.

If the issue persists after mechanical and parameter checks, provide:

I’ll help narrow it down further.

The Siemens fault code A07991 (often searched as 0a7991) specifically indicates that Motor Data Identification is activated. This is not a critical equipment failure but rather a status alarm triggered during the commissioning or configuration phase of Siemens SINAMICS drives, such as the G120, G120C, and G150 series. Understanding Alarm A07991

When this code appears, the drive is notifying you that a motor identification routine has been scheduled or is currently running. This routine is essential for the inverter to learn the electrical characteristics of the connected motor (such as resistance and inductance) to ensure precise control.

Behavior: The alarm typically disappears automatically once the identification routine completes successfully.

Restriction: While this alarm is active—especially if a rotating measurement is selected—you may be unable to save parameter assignments to the ROM until the process finishes. Common Troubleshooting Steps

If the alarm does not disappear or prevents the drive from running, use the following methods to resolve it: 1. Complete the Identification Routine

The most direct fix is to allow the drive to perform the check. siemens fault 0a7991

Apply a power-on command (start signal) to initiate the routine.

Ensure the motor is safe to move if a "Rotating Measurement" was selected. Once finished, the alarm will clear itself. 2. Manually Disable the Routine

If you do not wish to perform the identification (e.g., during bench testing without a motor), you can disable it via the Siemens Parameter List:

Set p1900 = 0 (Motor data identification and rotation test: Disabled).

Set p1300 = 0 (Control mode: V/f standard) if advanced vector control isn't needed immediately. 3. Resolving Persistent Alarms (A07991 + A8526)

This alarm frequently appears alongside A8526, which indicates a communication timeout with a PLC. While the exact wording may vary slightly depending

Check Connections: Ensure the Profinet/Profibus cable is securely plugged in and the PLC is powered on.

No PLC Connection: If testing the drive standalone, set p2030 = 0 (No protocol) and perform a "RAM to ROM" save. Technical Summary Table Default/Action Description p1900 Set to 0 Disables the motor identification routine. p0971 Set to 1

Saves parameters from RAM to ROM (perform after clearing alarm). p0340 Set to 0

Removes the requirement for motor details for initial testing.

If you are performing a Factory Acceptance Test (FAT) without the final motor, attaching a smaller spare motor can sometimes bypass identification faults by providing enough feedback for the drive to initialize. Alarms A 7991 and A8526 - SiePortal - Siemens

Either the network connection (profibus/profinet?) between the PLC controlling the drive is powered off, network connection lost ( Siemens SiePortal How can resolve the alarm Code A7991 - SiePortal - Siemens "I/O access error when updating the process image

If the fault is intermittent: