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66-42

Code 66-42 VFD Control Relay Shorted Lockout

Carrier · Air Conditioners & Heat Pumps

What does 66-42 mean?

The system's Variable Frequency Drive (VFD) control relay coil is detected as shorted to ground, making the system inoperable. The VFD will not be powered.

Common Causes

  • Damaged communication harness shorting to ground or another wire.
  • Internal short within the VFD control relay.
  • Problem within the VFD itself.

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Turn off power to the unit at the circuit breaker and wait 30 seconds before turning it back on.

  2. 2

    Inspect the communication harness between the PCM and VFD for any signs of damage, pinching, or exposed wires.

  3. 3

    Check to ensure all connections on the communication harness are tight and secure.

  4. 4

    If the fault persists, a professional should test the resistance of the harness and VFD components.

Browse all codes in this manual (60)
Verified technical data. Last updated: April 2026

Related Faults

14-94Code 14-94 Line Voltage Low — Incoming voltage is too low15-94Code 15-94 Line Voltage High — Incoming voltage is too high17-06Code 17-06 Lost BTM Communication — Bluetooth module not communicating with control board
Source: Carrier Air Conditioners & Heat Pumps
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