Danfoss VFD Warning 24 or W24 External Fan Fault

Getting Warning 24 (W24) on your Danfoss VLT drive? Learn what causes the external fan fault, how to troubleshoot seized bearings, and steps to restore cooling.

Danfoss VFD Warning 24 or W24 External Fan Fault


If your Danfoss VLT drive is flashing Warning 24 (W24), it is sending out a distress signal regarding its cooling system. Specifically, the drive has detected that the external cooling fan—the one responsible for pulling air through the main heatsink—is either not spinning, spinning too slowly, or disconnected.

While a "Warning" won't always stop the motor immediately (unlike an "Alarm"), ignoring Warning 24 is dangerous. Without proper airflow, the internal power modules (IGBTs) will quickly overheat, eventually leading to a critical Alarm 69 (Overtemperature) and a complete system shutdown. Here is how to diagnose and fix the issue before it stops your production.

Why Does Warning 24 Trigger?

The Danfoss drive monitors the fan’s feedback signal or current draw to ensure it is operational. Common causes for this warning include:

  • Physical Blockage: Dust, lint, or manufacturing debris has physically jammed the fan blades.
  • Bearing Failure: Over time, the fan bearings dry out and seize, preventing the fan from reaching the required RPM.
  • Loose Connections: The fan's power or feedback connector has wiggled loose due to cabinet vibration.
  • Fan Capacity Setting: In some cases, the drive is programmed to look for a fan that isn't physically installed (often after a control card swap).

Step-by-Step Troubleshooting Guide

Safety Warning: VFDs contain high-voltage capacitors. Always disconnect the main power and wait at least 15 minutes for the charge to dissipate before opening the drive or touching internal components.

1. The "Manual Spin" Test

With the power safely disconnected, locate the external cooling fan (usually at the bottom or top of the drive depending on the frame size).

  • Use a thin, non-conductive tool to gently flick the fan blades.
  • Healthy Fan: It should spin freely for a second or two.
  • Faulty Fan: If it feels stiff, "crunchy," or doesn't move at all, the bearings have failed. The fan must be replaced.

2. Clear Debris and Dust

In environments like textile mills or woodworking shops, the heatsink and fan can become clogged easily.

  • Use dry, low-pressure compressed air to blow out the fan housing and the heatsink fins.
  • Check the air intake and exhaust vents on the cabinet door. If the filters are clogged, the fan has to work harder, which can trigger a current-related warning.

3. Inspect the Fan Connector

Vibration is the "silent killer" of industrial connections.

  • Locate the fan's electrical plug (typically a small 2-pin or 3-pin connector).
  • Unplug and re-seat the connector firmly.
  • Check the wires for any signs of pinching or fraying.

4. Verify Parameter Settings

If you have recently replaced the fan or the control board, the drive might be looking for the wrong fan type.

  • Check Parameter 14-53 (Fan Monitor). This parameter tells the drive how to react to a fan failure. Ensure it is set according to your operational needs.
  • If you have installed a new fan that is not an original Danfoss part, the drive may not recognize the feedback signal, causing a persistent Warning 24.

Summary: When to Replace the Fan?

VFD cooling fans are considered "wear items." They typically have a lifespan of 30,000 to 50,000 hours. If your drive is more than 4–5 years old and is showing Warning 24, the most reliable fix is to replace the fan assembly entirely. Always use the exact part number listed in the Danfoss spare parts manual for your specific frame size to ensure the RPM feedback matches the drive's expectations.

Action Check For
Visual Check Obstructions, dust "carpets," or loose wires.
Mechanical Test Bearings should be silent and rotate smoothly by hand.
Voltage Test Verify 24V (or rated voltage) is reaching the fan plug.
Parameter 14-53 Ensure fan monitoring is enabled if a fan is present.

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