Troubleshooting Danfoss VFD Internal Fault

Danfoss drive showing an "Internal Fault"? Learn how to troubleshoot VFD processor errors, memory issues, and hardware failures, and when it’s time to replace the drive.

Troubleshooting Danfoss VFD Internal Fault


When your Danfoss VLT drive displays an "Internal Fault" or a high-number alarm (typically in the range of Trip 38 to Trip 80), it is reporting a failure within the drive’s own "brain"—the main processor, the memory, or the internal communication bus between boards.

Unlike an Overcurrent (Alarm 13) or Earth Fault (Alarm 45), which are caused by the motor or external environment, an Internal Fault is a signal that the drive’s hardware or firmware has encountered a condition it cannot recover from. Here is how to approach this serious alarm.

Common "Internal Fault" Codes

  • Trip 38: Internal fault (often related to internal gate drive circuits).
  • Trip 39: Internal fault (memory/checksum error).
  • Trip 80: Initialization error (often occurs after a manual reset/init).
  • Other High-Number Alarms: Usually point to communication failures between internal PCBs (Power Card vs. Control Card).

Step-by-Step Troubleshooting Guide

1. Perform a Controlled Power Cycle

Internal faults are sometimes caused by transient power spikes that "confuse" the drive’s processor.

  1. Remove all power (including external 24V DC control power).
  2. Wait at least 15 minutes. This allows the internal control board capacitors to discharge completely.
  3. Reapply power and see if the drive clears the fault.

2. Check for External Interference

Sometimes, high-frequency electrical noise from nearby equipment can corrupt the drive’s internal signal lines.

  • Ensure all control signal cables (4-20mA, digital inputs, etc.) are shielded and that the shields are grounded correctly.
  • Check for loose connections at the drive's control terminals, as a floating signal can cause unexpected processor behavior.

3. Manual Initialization (The "Last Resort")

If the drive remains stuck in an internal fault, you can try to "reboot" the firmware logic by performing a Manual Initialization. Warning: This will reset all your parameters to factory defaults.

  1. If you have a backup, save your parameters to the LCP (keypad) using Parameter 0-50.
  2. Follow the procedure for your specific VLT series to perform a manual initialization (usually involving specific button combinations on power-up).
  3. After the drive initializes, it should return to a "Ready" state. If the fault returns immediately, the hardware is physically damaged.

4. Inspect Internal Hardware

If the drive has been in service for many years, internal components may be failing:

  • Visual Inspection: If you are authorized to open the drive, look for burnt components or "blown" capacitors on the control card or power card.
  • Environment: If the drive is in a dusty, oily, or high-humidity environment, contaminants can cause microscopic shorts on the control board, leading to processor errors.

Is the Drive Repairable?

Most Internal Faults (Trip 38–80) are not repairable in the field. If a power cycle and initialization do not clear the error, it indicates that the main control card or the connection between boards has suffered a permanent failure.

For modern industrial applications, an internal fault usually signals the end of the drive’s service life. Replacing the drive is the most reliable way to ensure the safety of your machinery and the stability of your production process.

Summary Advice

If your Danfoss drive is showing an internal fault, do not try to bypass it. These faults exist to protect you from the drive behaving unpredictably (e.g., commanding the motor to start or run at max speed without permission). If the fault persists, replace the drive unit.

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