S7-400H CPU 414-H All LEDs Blinking Defective Mode

S7-400H CPU 414-H All LEDs Blinking: Defective Mode Troubleshooting

The SIMATIC S7-400H fault-tolerant controller family uses redundant CPUs that continuously cross-check each other. When the standby (slave) CPU enters the DEFECTIVE operating state, every status LED on the front panel flashes in unison, the diagnostic buffer records an internal checksum error, and the controller drops out of redundant operation. This article explains how to read the diagnostic events, isolate the root cause, verify the memory-card configuration, update the firmware, and replace the CPU in an S7-400H system based on the CPU 414-4H (medium performance range) and the related CPU 412-3H / CPU 417-4H variants.

1. Problem Description

Field report: an S7-400H rack contains a primary CPU in rack 0 and a secondary (standby) CPU in rack 1. The standby CPU enters a fault state with all front-panel LEDs flashing simultaneously at approximately 2 Hz. STEP 7 / PCS 7 displays the following diagnostic buffer entries:

Per the SIMATIC S7-400H System Manual, simultaneous flashing of all LEDs on a CPU 41x-H is the unambiguous hardware indicator for the DEFECTIVE state. The CPU has detected an internal error that it cannot clear by itself; it has intentionally disabled all process I/O update functions and will not accept a START/RUN command from STEP 7 until the underlying fault is removed.

2. Operating Modes of an S7-400H CPU

An S7-400H CPU moves between the following modes. The diagnostic entry "Requested operating mode: DEFECTIVE" indicates the transition arrow shown in bold below.

The DEFECTIVE state is intentionally sticky: even if the internal cause is transient, the CPU will not return to RUN on its own. You must perform a power cycle after fixing the underlying cause, and in many cases the CPU must be replaced.

3. Root Cause Analysis

From the diagnostic buffer and the field history, the most likely root causes for a CPU 414-4H (or 412-3H / 417-4H) entering DEFECTIVE with a checksum error are listed in order of frequency observed in installed bases.

4. Memory Card Requirements: S7-400 vs S7-300

The most common cause of a "spare CPU with all LEDs flashing" is a confusion between S7-300 and S7-400 memory cards. S7-400 CPUs do not accept MMC (Micro Memory Card); they require MC 952 modules in either RAM or FEPROM technology.

The CPU 414-4H has only 512 or 1024 kB of internal load memory, which is insufficient for any realistic H-system program including the S7-H Communication blocks. If no card is inserted, the CPU cannot load the project from EPROM, and it transitions to DEFECTIVE on power-up.

5. Memory Card Catalog Numbers

The following SIMATIC MC 952 part numbers are the most common cards used with the CPU 41x-H family. All numbers are 6ES7 952-1xxx-0AA0 series. Confirm the voltage variant against the CPU manual before ordering.

6. Front-Panel LED Reference for CPU 41x-H

The CPU 41x-H front panel provides the following LED indicators. Reading them in combination is the fastest way to differentiate a DEFECTIVE state from a normal redundancy failover.

7. Step-by-Step Recovery Procedure

7.1 Prerequisites

1. A PG/PC with STEP 7 V5.5 SP4 (or later) or TIA Portal V13+ and an Ethernet or MPI/DP cable to the CPU.
2. The original S7-400H project, archived, with the most recent System Data and Hardware Configuration.
3. A compatible MC 952 memory card (RAM or FEPROM) with sufficient capacity for the project.
4. Spare CPU of the exact same order number (MLFB), for example 6ES7414-4HM14-0AB0 for a CPU 414-4H.
5. The correct firmware update file for the H-CPU, downloaded from the Siemens Industry Online Support portal (see section 8).
6. Access to the relevant Siemens manuals:
   • New S7-400H CPU for the Medium Performance Range: CPU 414-4H

7.2 Confirm the State

1. Open STEP 7 → Accessible Nodes or the online view of the H-station.
2. Select the standby CPU (rack 1) and read the diagnostic buffer: PLC → Diagnostic/Setting → Diagnostic Buffer.
3. Confirm Event 1 / 1500 is a checksum error, "Requested operating mode: DEFECTIVE".
4. Look at the LEDs: all flashing in unison at 2 Hz indicates DEFECTIVE.

7.3 Check the Memory Card

1. Power down the standby CPU only (do not de-energize the master CPU; in an S7-400H, the master must keep controlling the process).
2. Remove the card from the CPU slot.
3. Visually inspect the card: check for cracked housing, oxidized contacts, missing label, and confirm it is a MC 952 module — not an MMC.
4. If the card is the wrong family (MMC) or the slot is empty, the CPU will go to DEFECTIVE on the next power-up regardless of any other fix.

7.4 Reload the Project onto a New Card

1. Insert the card into the PG's prommer (or use the CPU's online card-flash function: PLC → Download User Program to Memory Card).
2. Select RAM if the CPU has a backup battery installed (recommended for H systems), or FEPROM if you want non-volatile storage without a battery.
3. Download the project. Verify in STEP 7 that the card's Used / Total memory matches the project size.

7.5 Firmware Update (Recommended)

1. Check the current firmware on the defective CPU: PLC → Diagnostic/Setting → Module Information → Firmware.
2. Go to Siemens Industry Online Support and search for "Operating System Update CPU 412-3H / CPU 414-4H / CPU 414-4H PG / CPU 417-4H". The current entry lists the latest V6.x firmware packages.
3. Match the firmware version to both CPUs of the H system; mixed firmware is not supported.
4. Run the update from STEP 7: PLC → Update Firmware. The CPU must be in STOP or unconfigured; the update takes approximately 5 to 15 minutes depending on the package.
5. Power cycle the CPU when prompted.

7.6 CPU Replacement

1. Take the master CPU offline only if you are prepared to lose the redundant backup during the swap. In most cases, leave the master in RUN and swap the standby.
2. Open SIMATIC Manager with the project online.
3. On the standby CPU, select PLC → Operating Mode → STOP.
4. Disconnect the sync fiber (IFC1/IFC2) and any PROFINET cables; mark them.
5. Power down the standby rack or remove the CPU from its slot.
6. Insert the spare CPU and the loaded MC 952 card.
7. Power up: the new CPU performs a self-test, then displays STOP with the MAST/STBY LEDs off (no role assigned yet).
8. Reconnect the sync fibers and PROFINET cables.
9. In STEP 7, select PLC → H-System → Link-Update, or trigger a redundancy update from the HMI / PCS 7 faceplate. The new CPU is assigned the standby role and begins to sync.
10. When the LINK LED is steady green on both CPUs, redundancy is restored.

8. Firmware Update Sources

Siemens publishes firmware update packages per CPU order number. For the CPU 414-4H and its H-system siblings, look in the Siemens Industry Online Support under "Product Support → Automation Technology → SIMATIC → PLC → S7-400 / S7-400H → CPUs → CPU 414-4H". The page referenced in the research lists the current medium-performance-range CPU and the supported firmware upgrade path.

• Entry: New S7-400H CPU for the Medium Performance Range: CPU 414-4H

Common firmware update file names follow the convention S7H4xx_Vxx_x.exe, for example S7H414_V06.exe for CPU 414-4H V6 firmware. Always match the MLFB (order number, e.g. 6ES7 414-4HM14-0AB0) to the update before running it.

9. Verification

After completing the fix, perform the following checks before declaring the H-system healthy.

1. LED verification: Master — RUN green, MAST yellow, LINK green. Standby — RUN green, STBY yellow, LINK green.
2. STEP 7 module information: PLC → Module Information → Diagnostic Buffer on the standby should show "H-system standby in sync", no DEFECTIVE entries.
3. Self-test: In SIMATIC Manager, PLC → H-System → H-Status reports Redundant operation: OK.
4. Failover test: Pull the master CPU's power. The standby should take over the process within the configured OB 72 / OB 80 reaction time (typically < 100 ms) and the system should remain in RUN-solo. Re-seat the master and verify that it returns as a new standby.
5. Card contents: Re-read the MC 952 with the PG and confirm the checksum of the downloaded block matches the project source.

10. Troubleshooting Matrix

11. S7-400H vs S7-300 Memory Card Pitfalls

Many field engineers keep S7-300 and S7-400 spare parts in the same cabinet. The following pitfalls are the most common sources of a DEFECTIVE state in mixed-stock environments.

12. Preventive Measures

1. Mark every spare CPU with the firmware version it last carried, and keep the project file in the same drawer.
2. Always store spare S7-400 CPUs with an MC 952 card inserted — even an empty FEPROM card prevents the DEFECTIVE state on first power-up.
3. Track Flash card service life: replace FEPROM cards every 10 years in continuous service, or after 100,000 write cycles.
4. Run a redundancy failover test at least once a year; log the H-status to the process historian.
5. Maintain identical firmware on both H-CPUs; never mix V4.x with V5.x, or V5.x with V6.x.
6. When decommissioning, write-protect the FEPROM card to prevent accidental overwrite during testing.