SF or BF LED illuminated?
Understand what the indicators suggest, the diagnostic buffer to read, and how to distinguish CPU, I/O and communication faults.
Follow the diagnostic guide →Start with a specific symptom
Changing an S7-300 CPU?
Check full order code, programme backup, MMC, firmware and rack details before agreeing a replacement.
Use the CPU checklist →Is your PLC nearing end of life?
Research published manufacturer lifecycle notices and plan critical spares before an emergency.
Explore PLC lifecycle information →Labelling wires and PLC I/O
Prepare printable wire, terminal and I/O labels from spreadsheet data with a free browser-based tool.
Open the label generator →How to use these guides: Identify the exact CPU/drive type and installation first. Diagnostic LEDs are clues, not proof that a CPU has failed. Do not bypass guarding, defeat safety systems, change a live programme or reset a machine to clear a fault without your site's authorised safe process.
Record these details before replacing a PLC
- Photograph all current LED states and note the time and machine symptom.
- Read and save the first relevant diagnostic events before a power cycle.
- Record the complete manufacturer order number, firmware, communications and I/O layout.
- Verify you have a usable programme and parameter backup, with approved restoration and restart procedures.
- If a component appears faulty, distinguish a confirmed failure from wiring, supply or network causes.
Need replacement stock, or have surplus controls?
AO supplies its own listed industrial automation stock and investigates obsolete part enquiries. We also assess surplus PLCs, drives and HMIs for trade purchase or managed resale. A guide is not a promise that a specific part is available.