What to check

At the door panel check that the conductor going to the 'C' or '-' terminal is securely connected.

This wire goes to the '-' terminal in the power supply so check the connection at that end too.

Check that the 'C'/'-' wire is not broken. If a spare wire is available join at one end and check continuity on the loop.

Corrective Action

Ensure terminals are firmly tightened and that the wire is stripped adequately and is making a good connection.

If your test shows a conductor is open circuit, substitute it with a spare conductor if available.