Power Transformer and Control Board Fuse Diagnostic

Test your furnace power transformer and control board fuse to ensure they are working normally.

Start here

In most furnaces the control board runs on low voltage AC power (24 – 28V), which is converted by a power transformer from the line voltage (120V in North American homes). A low voltage fuse on the control board regulates power.

If the power transformer fails or control board fuse trips, the furnace will appear to have no power supply — there will be no power to the control board and no LED status lights.

Refer to the furnace glossary if you need help identifying furnace parts or terms.

Testing the power transformer

Once you’ve done basic troubleshooting and found there is no power to the furnace or control board, use this procedure to determine if the power transformer has failed.

Tools needed

  • standard multimeter

Testing procedure

  1. Turn off the power to the furnace.
  2. Remove the lower furnace cabinet door to expose the control board.
  3. Locate the power transformer on the control board. There will be two white and black wires that feed line voltage to the power transformer.
  4. Locate the other two coloured wires (not black and white) that come off the power transformer and plug into the control board. These are the low voltage supply wires.
  5. Trace the low voltage supply wires back to the control board and make note of where they connect (or take a photo).
  6. Disconnect the low voltage supply wires from the control board. They typically have female spade connectors that are friction-fit to the control board.
  7. Set your multimeter to measure AC voltage.
  8. Connect one multimeter probe to each wire using the connectors. Ensure the connectors and probes are not in contact with any metal surfaces.
  9. Turn on the power to the furnace and depress the lower furnace cabinet door switch (as it would be when the cabinet door is installed).
  10. Record the voltage measured by the multimeter.
  11. Turn off the power to the furnace.
  12. Reconnect the low voltage wires to the control board in the same manner you found them.

Interpreting the results

Measured 24-28V AC

The power transformer is working properly and delivering power to the control board. Go to the next section and test the control board fuse.

No voltage measured

The power transformer has failed and needs to be replaced.

Testing the control board fuse

Once you’ve determined that the power transformer is working properly, use this procedure to determine if the control board fuse has blown or if the control board has failed.

Testing procedure

  1. If you’ve reinstalled it, remove the lower furnace cabinet door to expose the control board.
  2. Turn on the power to the furnace and depress the lower furnace cabinet door switch (as it would be when the cabinet door is installed).
  3. Confirm that the control board is not receiving power and no LEDs are on. (If there is power and LEDs are coming on, stop this procedure and check the error code.)
  4. Turn off the power to the furnace and release the lower furnace cabinet door switch.
  5. Locate the control board fuse or breaker, which should be one of the following:
    • an automotive-style fuse on the control board (most furnaces)
    • a breaker switch (3 – 5A) near the cabinet door switch (newer furnaces)
  6. Inspect the fuse or breaker to see if it has been blown or tripped. If there is a fuse, pull it out to inspect it.
  7. If there is a fuse and it has blown, replace it with a new fuse. If there is a breaker and it has tripped, reset it.
  8. Turn on the power to the furnace and depress the lower furnace cabinet door switch (as it would be when the cabinet door is installed).
  9. Confirm if the control board is receiving power and status LEDs are on.

Interpreting the results

If replacing the fuse or resetting the breaker does not result in the control board powering up, it is likely that the control board has failed and should be replaced.

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Furnace repairs should only be performed by a certified technician.

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