S7-400 CPU Stays in STOP After Power Cycle: Causes and Fix

1. Problem Statement: S7-400 CPU Returns to STOP After Power Restoration

The reported failure mode: a SIMATIC S7-400 CPU 414-2 (order number 6ES7414-2XK05-0AB0) is running in RUN mode. The 24 V DC system supply is removed. When the supply is re-applied, the CPU goes through its power-on sequence and ends up in STOP rather than RUN. The mode selector remains in the RUN position, no SF (group fault) LED is lit, and the diagnostic buffer shows no programming or I/O error. The operator must toggle the mode selector from RUN to STOP and back to RUN to bring the program back online. No fault is reported by the user program; the program simply does not resume on power-on.

This symptom is characteristic of a CPU that cannot perform a warm restart (Wiederanlauf) on power restoration and is forced into a complete restart / cold restart (Kaltstart, Neuanlauf). The most frequent cause in field service is a missing, exhausted, or loose backup battery. The case in the source thread was resolved by reseating the battery, not by replacing it.

2. Affected Hardware and Catalog Identification

The reported configuration uses a standard S7-400 mid-range CPU with a flash memory card. Confirm the exact catalog number from the front-panel label and the firmware version in the diagnostic buffer before ordering spares. The family of order numbers below the "-0AB0" suffix and the "-XK05" mark are decoded in the SIMATIC S7-400 S7-400 Automation System, CPU Specifications manual.

For full mechanical, electrical, and ordering data, refer to the S7-400 CPU Specifications manual on the Siemens Industry Online Support portal.

3. Root Cause: Loss of Retentive Backup

The S7-400 CPU preserves a portion of its state across a power loss through a lithium backup battery. The battery powers an SRAM area that holds:

• Retentive bit memory (M, MB, MW, MD, and the first 16 bytes of MB0..MB15 by default)
• Retentive S7 timers (T) and counters (C) declared as retentive in the configuration
• Retentive instances of FBs declared with the "non-retain" attribute disabled
• The last operating mode and the "last transition" status (RUN -> STOP, RUN -> power-fail)

If the battery is missing, exhausted, or has a poor contact, the CPU loses all of the above on power-down. On the next power-on the firmware detects the missing data and refuses to perform a warm restart. It executes a complete restart (cold restart) and stops at the end of OB 102 initialization, awaiting operator acknowledgement.

The flash memory card (6ES7952-0KH00-0AA0) keeps the project itself safe (blocks, hardware configuration, symbols) but does not retain runtime values. A flash card is therefore necessary but never sufficient to recover RUN mode automatically. This is the most common misconception in field service.

4. S7-400 Startup Modes

The S7-400 distinguishes three startup types. The selection depends on backup state and STEP 7 / TIA Portal hardware configuration. Compare the table below with the S7-400 CPU Specifications manual for the exact behavior of your firmware version.

The relevant STEP 7 / TIA Portal setting is in the CPU properties, Startup tab. The radio options are:

1. Warm restart - manual: After power-on, the CPU enters STOP and waits for the operator to toggle the mode switch to RUN. Requires a healthy battery.
2. Warm restart - automatic: After power-on, the CPU executes OB 101 and transitions to RUN without operator action. Requires a healthy battery.
3. Complete restart - manual / automatic: Same as above, but the warm restart is replaced with a cold restart (OB 102). This option is the only auto-run fallback when the battery is exhausted.

Default in STEP 7 and TIA Portal for new S7-400 projects is "Warm restart - manual". This is the most common reason why a healthy, recently-installed CPU still requires a manual mode-switch toggle after a power cycle.

5. Memory Card Function in the S7-400 Power-On Sequence

Every S7-400 CPU exposes a slot for a memory card on the front panel. The card acts as an extension of the load memory and is the only non-volatile program storage on the CPU. Detailed information is published in the Siemens TIA Portal help:

What you should know about memory cards for S7-400

Key points for the auto-run question:

• The flash card preserves the project (blocks, symbols, HW configuration) across power-off and battery loss. Without a card, the project is lost when the battery is exhausted; the card is therefore mandatory for unattended sites.
• The card does not retain the work memory, the last operating mode, or any runtime values. It does not, by itself, enable warm restart.
• When the card is present and the battery is healthy, the power-on sequence copies project blocks from the card into the work memory, checks their CRC, then triggers the configured startup type.
• Three card types exist: RAM (volatile, battery-backed, allows online editing), FLASH (non-volatile, cannot be edited online), and EPROM (non-volatile, programmed externally with a Prommer). FLASH is the most common choice for production CPUs and the one used in the reported case.
• The card type is detected automatically. Inserting a FLASH card over an existing RAM card requires a manual reload of the project to update the on-card content.

6. Battery Backup System and Service Life

The backup battery is the deciding component. The typical part numbers for an S7-400 rack are listed below. Always order by the exact order number printed on the existing cell or on the parts list in the CPU manual.

Service life is shortened by:

• High ambient temperature (derate by ~50% per 10 °C above 25 °C)
• Long storage time of the CPU before commissioning (battery discharges even with the CPU off)
• Repeated deep discharge (long power-off periods without a PS battery)
• Loose contact in the holder (intermittent high-resistance path)

The CPU monitors the battery voltage and writes a warning event into the diagnostic buffer when the voltage drops below ~2.7 V. The BATF LED on the front panel lights up. After this warning, the user has typically a few weeks of buffer time before the CPU drops the work memory.

7. Mode Switch Behavior

The S7-400 mode selector is a 4-position key switch on the front panel. The positions and their effect on startup are:

For automatic restart, the switch must be in RUN or RUN-P. If the switch is in STOP or MRES at the moment of power restoration, the CPU will not enter RUN automatically regardless of the configuration. Confirm the switch position as the first step in any "no auto-run" troubleshooting case.

8. Reading the Diagnostic Buffer for Confirmation

Before touching the hardware, read the CPU diagnostic buffer online. This is the most reliable confirmation that the root cause is the battery and not, for example, an I/O fault during startup, a missing hardware configuration, or a programming error.

Procedure with STEP 7 V5.x:

1. Open SIMATIC Manager and the project.
2. Select the CPU in the project tree.
3. PLC > Online > Accessible Nodes (or use the established online connection).
4. PLC > Diagnostics / Setting > Diagnostic Buffer.
5. Inspect the most recent entries, in particular the events around the last power-on.

Procedure with TIA Portal V16 and later:

1. Open the project, select the S7-400 device.
2. Click Go online and accept the online connection.
3. In the project tree, Online & diagnostics > Diagnostic buffer.
4. Filter for "Power" or "Battery" events.

Common event IDs to look for:

A pattern of 0x4311 (Cold restart) -> 0x4308 (STOP) -> 0x4300 (Power ON) across several power cycles is the smoking gun for the battery fault. If the diagnostic buffer is empty, the CPU is unable to enter operational state at all; the cause is more likely a missing or corrupted memory card.

9. Step-by-Step Resolution

The reported case was resolved by reseating the battery, not by replacing it. Follow the procedure below, which combines inspection, reseating, reconfiguration, and verification.

Step 1: Capture the current state

Connect to the CPU online, download the diagnostic buffer, and export the hardware configuration. Note the BATF LED state, the mode switch position, and the firmware version.

Step 2: Open the front cover and locate the battery holder

Power the PLC down (or, on S7-400, perform the battery swap with the power on, within the buffer time window). Open the front cover. The battery holder is typically at the bottom of the CPU or on the PS module, in a slot labelled "BATT".

Step 3: Inspect the holder

• Check the spring contacts for oxidation, deformation, or loss of tension.
• Check the orientation of the cell: the polarity is marked on the holder and the cell.
• Check the connector cable to the PS / CPU board; on some PS types the battery is wired to the PS, not the CPU.

Step 4: Reseat the cell

Remove the cell, clean the contacts with a dry eraser or isopropyl alcohol, and reinstall it. Confirm the cell snaps into the holder. This step alone resolved the original case in the source thread.

Step 5: Replace if voltage is low

Measure the cell voltage under load (use a 100 Ω resistor in parallel to simulate the CPU drain). A healthy cell reads > 3.4 V. Below 3.0 V the cell is exhausted and must be replaced. Use only the approved Siemens types listed in section 6.

Step 6: Power on and verify

Apply 24 V DC. Watch the CPU LEDs in this order:

1. SF / BF off (no fault)
2. DC5V on (backplane powered)
3. FRCE on if forced jobs are active
4. RUN on within a few seconds

If the CPU now goes to RUN automatically after power-on, the problem is solved. Re-check the diagnostic buffer; a "Warm restart" event should be present at the top.

10. Enabling Automatic Restart in STEP 7 / TIA Portal

If the battery is healthy but the CPU still requires operator intervention, the cause is the STEP 7 startup configuration.

STEP 7 V5.x

1. HW Config > open the CPU properties > Startup tab.
2. Set Startup on power on: Warm restart - automatic (battery must be serviceable).
3. Alternatively, set Complete restart - automatic if you need a guaranteed cold start on every power-on.
4. Save, download, and perform a STOP-to-RUN transition to make the new settings active.

TIA Portal V16+

1. Project tree > Devices & networks > select the CPU.
2. Properties > Startup.
3. Set the same Startup on power on drop-down.
4. Compile and download.

11. Verification Procedure

After the fix, perform at least three cold power cycles to confirm the result. Document each cycle in the maintenance log.

1. Cycle 1: Power off for 10 s, power on. CPU should reach RUN within 5 s.
2. Cycle 2: Power off for 60 s, power on. Same behavior.
3. Cycle 3: Power off for 5 min, power on. Same behavior.

For each cycle, capture the diagnostic buffer and confirm a 0x4310 Warm restart event. If a 0x4311 Cold restart re-appears, the battery is marginal even though it measures > 3.0 V on the bench. Replace it.

Optional acceptance test: force a retentive bit (M 100.0) to TRUE, perform a power cycle, and verify that M 100.0 is still TRUE in the online watch table. This confirms that the warm restart is preserving retentive memory.

12. Preventive Maintenance and Best Practices

• Replace S7-400 backup batteries every 24 months as a preventive measure, regardless of the BATF LED state.
• Mark the battery installation date on the cell with a permanent marker.
• Keep at least one spare cell per CPU on the cabinet shelf.
• For unmanned sites, configure a battery-low alarm via the OB 81 (I/O fault) or via a CP 443-1 email / SMS push.
• Use the optional 2-cell holder (e.g. 6ES7971-1AA00-0AA0) for redundant battery capacity where supported.
• Always pair any battery change with a diagnostic buffer download for the maintenance record.
• Avoid mixing old and new cells in multi-cell holders; replace both at the same time.
• Verify the mode switch position in the maintenance log to prevent accidental STOP position during power-on.

13. Related S7-400 CPU Models

The same root cause and remediation apply to the entire S7-400 mid-range and high-range CPU family. The catalog numbers differ but the diagnostic events and the battery slot location are consistent.

14. Frequently Asked Questions