Siemens S7-300 CPU Module Failure Diagnosis: 5 Steps for 6ES7315-2AH14-0AB0

Your Siemens S7-300 CPU – specifically the 6ES7315-2AH14-0AB0 Central processing unit – just stopped working. The production line is down. Where do you start?

This guide provides accurate diagnostic steps based on the official specifications of the 6ES7315-2AH14-0AB0 CPU 315-2DP.

*Before you begin: This CPU requires a 24V DC power supply (19.2V – 28.8V) and draws approximately 850 mA. It uses a Micro Memory Card (MMC) for program storage. No integrated PROFINET – only MPI and PROFIBUS DP.

Step 1: Check the LED Status Indicators

The 6ES7315-2AH14-0AB0 has a specific LED layout. Here's what each light means:

Typical fault combinations:

SF + STOP: Program error or MMC card issue

BF + SF: PROFIBUS configuration problem

DC 5V OFF: Internal power supply failure (replace CPU)

*If SF is on and STOP is on, the CPU cannot start. Check the MMC card first.

Step 2: Check the Power Supply

The 6ES7315-2AH14-0AB0 requires a stable 24V DC supply.

What to do:

1. Use a multimeter to measure voltage at terminals L+ and M on the front connector
2. Acceptable range: 19.2V – 28.8V DC
3. Check that the supply can deliver at least 850 mA (plus load from output modules)

Common power issues:

• Voltage below 19.2V → CPU may not start or behave erratically
• Voltage above 28.8V → Risk of permanent damage
• No voltage → Check external fuse, wiring, and power supply unit

*If DC 5V LED is off but L+ has 24V, the CPU's internal power converter is faulty – replacement needed.

Step 3: Diagnose Communication Faults (PROFIBUS DP / MPI)

The 6ES7315-2AH14-0AB0 has no PROFINET. It uses:

MPI (Multi-Point Interface) – for programming and HMI

PROFIBUS DP (RS-485) – for distributed I/O and drives

If BF (Bus Fault) LED is on:

1. Check PROFIBUS connectors – are they properly seated and terminated?
2. Verify the PROFIBUS cable – no breaks, no short circuits
3. Check if the bus termination resistor is activated at both ends
4. Verify the PROFIBUS address settings in hardware configuration
5. Test with only the CPU on the bus – if BF goes off, a slave device is faulty

If you cannot communicate via MPI:

1. Check the programming cable (PC Adapter or CP5711)
2. Try a different MPI address (default is 2)
3. Reset the CPU to factory settings (requires MMC card removal – see Step 4)

*A solid BF light often means the CPU cannot find its configured PROFIBUS slaves. If all slaves are powered on and connected, the issue may be a faulty CPU port.

Step 4: Check the Micro Memory Card (MMC)

The 6ES7315-2AH14-0AB0 requires an MMC card to store the program. No program = CPU stays in STOP.

What to check:

*Important: The 6ES7315-2AH14-0AB0 does NOT work with standard SD cards. Only Siemens Micro Memory Cards are supported.

To test without an MMC card:

Remove the MMC and power cycle the CPU. It should go to STOP (no SF if no hardware fault). If SF is still on with no MMC, the CPU may have internal damage.

Step 5: Test with a Spare Module

The fastest way to confirm a faulty CPU is to replace it with a known-good spare.

What to do:

1. Power off the system completely
2. Remove the MMC card from the suspect CPU
3. Insert the MMC card into a spare 6ES7315-2AH14-0AB0 (or compatible S7-300 CPU)
4. Install the spare CPU and power on

Results:

• Problem solved → Original CPU is faulty. Replace it.
• Problem remains → Issue is elsewhere (wiring, backplane, power supply, or external devices)

*No spare? Swan Electric keeps 6ES7315-2AH14-0AB0 Central processing unit in stock – new and original, ready to ship.

Quick Reference: 5-Step Diagnosis for 6ES7315-2AH14-0AB0

Step                                           Key Checkpoints

1.LED status                             SF+STOP = program/MMC issue; BF = PROFIBUS issue

2.Power supply                         24V DC ±20%, at least 850 mA

3.Communication                      PROFIBUS cable, termination, slave addresses

4.MMC card                              Inserted properly, compatible, not corrupted

5.Spare module test                   Replace CPU – does the fault follow?

What If You Need a Replacement 6ES7315-2AH14-0AB0?

Swan Electric stocks new and original Siemens S7-300 CPUs and modules.

Available now:

All modules are new and original. No refurbished, no counterfeit.

Still Having Trouble Diagnosing Your 6ES7315-2AH14-0AB0?

Our engineers can help you remotely. Send us photos of the LED status and MMC card.

Phone: +8615327455845

Email: lizzie@swanautomation.com

Swan Electric – New and original Siemens PLC modules, in stock, with expert support.