SOLVED: Power light blinks twice, fans high – Power Mac G5

If a user is experiencing an issue with their Power Mac G5 where the power light blinks twice and the fans ramp up to high speed immediately upon startup, it’s a clear indication of a hardware-related problem. This specific behavior—two blinks of the power LED followed by high-speed fans—usually signals a system diagnostic or thermal management issue. Thankfully, the issue has been well-documented, and with the right steps, it can be resolved.

When the Power Mac G5 encounters hardware issues at boot, it often communicates the problem through diagnostic LED flashes. Two flashes typically refer to a memory-related issue, but the high fan speeds indicate that the system isn’t detecting all required thermal or environmental sensors. This is commonly caused by a missing or failing component that the system depends on to monitor internal temperatures and environmental conditions.

Understanding the Symptoms

• Power light blinks twice: Indicates a memory or configuration error at startup.
• Fans blowing at high speed: Suggests the system is not receiving correct input from heat sensors or essential system modules.

In most documented cases, the root cause of this issue lies in a missing or disconnected part called the CMS—Controller Management System—more widely recognized as the Apple Service Diagnostic dongle or the front memory bay air deflector.

How to Fix the Issue

To resolve the problem, the user will need to ensure that the following steps are followed correctly:

1. Power Down the System: Disconnect the power cord and wait a few minutes. Always work with electronics unplugged to prevent electric shock.
2. Open the Side Panel: Remove the clear plastic cover inside the Power Mac G5 chassis. This panel is essential, as it helps regulate airflow and proper cooling.
3. Check for the Air Deflector: Many users miss replacing the internal plastic air deflector after working on their systems. Without it, sensors don’t operate properly, and fans speed up in response.
4. Reseat RAM Modules: Remove the memory sticks and re-insert them, ensuring they are properly seated. Try booting with one stick at a time if problems persist.
5. Inspect for Corrosion or Dust Accumulation: Remove dust buildup that can interfere with thermal and sensor connections. A blast of compressed air can help with this.
6. Reassemble and Test: Once everything is cleaned, checked, and properly in place, close the system and power it on again.

In virtually all successful resolutions of this issue, either reseating the RAM or replacing the missed clear plastic air deflector resolved the problem. The design of the G5 is specific in its need for airflow routing components—even seemingly minor parts are critical to correct operation.

Why the Air Deflector is Important

The internal air deflector is a vital component that directs airflow over the memory and CPUs. Without it, the system does not detect appropriate air movement or thermal conditions and, in response, ramps fans to full speed to prevent overheating. Apple designed the Power Mac G5 with thermal zones that rely on this part for sensor feedback.

Additional Tips

• If the clear air deflector is missing, consider sourcing a replacement from online marketplaces or a local Apple repair shop.
• Use Apple’s official Service Diagnostic Tool (if available) for in-depth error reporting.
• Ensure the system’s PRAM and PMU are reset if issues continue.

Conclusion

A Power Mac G5 experiencing blinking power lights and loud fans is often reacting to missing components or misconfigured RAM. By ensuring all thermal components are in place and memory is correctly installed, users can often bring these classic machines back to a stable state with minimal investment.

FAQ

• Q: What do two blinking power lights mean on a Power Mac G5?
  A: It usually indicates a memory configuration or detection error at startup.
• Q: Why are the fans on my Power Mac G5 running so loudly at startup?
  A: This typically happens when internal sensors or airflow components like the air deflector are missing or misaligned.
• Q: Can I operate my G5 without the clear plastic air deflector?
  A: No, the air deflector is necessary for proper airflow and sensor readings. Its absence will trigger high fan speeds and may cause thermal shutdowns.
• Q: How can I reset the PMU on a Power Mac G5?
  A: Unplug the system, then press and hold the PMU reset button located on the motherboard for a few seconds, followed by reconnecting power and restarting.