Hey there! I totally get the pressure you're under - a mysterious PLC error shutting down production every 3 hours with no OEM support and the threat of another line stoppage hanging over you. That's a maintenance technician's nightmare! Here's a systematic approach I'd recommend:

1. Document Everything First - Write down the exact error code, time stamps, and what was happening right before each shutdown. Look for patterns - is it always during a specific operation or load condition?

2. Check the Basics - Start with power supply stability (use a multimeter to check for voltage drops or spikes), wiring connections, and grounding. The 3-hour pattern might suggest something like thermal buildup or cyclic loading.

3. Access PLC Diagnostics - If you have programming software, connect to the PLC and check the diagnostic LEDs and error logs. Many PLCs store detailed fault information that can give you clues about whether it's a hardware fault, memory issue, or communication problem.

4. Monitor Live Data - Watch the PLC's input/output status and program execution in real-time as you approach the 3-hour mark. Look for any unusual patterns or values that might be triggering the fault.

5. Isolate the Problem - Since it's happening every 3 hours, try to identify if it's related to specific equipment cycles, temperature changes, or electrical loads. Check if any other equipment has similar timing patterns.

6. Check for Intermittent Issues - Look for loose connections, worn components, or environmental factors (temperature, vibration, humidity) that might be causing intermittent failures.

7. Research the Error Code - Even without OEM support, you can often find error code documentation online or in maintenance manuals. Try searching the exact error code with your PLC model number.

The key is to be methodical and document everything. Start with the simplest explanations first - power issues, loose connections, or environmental factors - before diving into complex programming or hardware problems. Good luck, and I hope you track down that pesky fault soon!