Lochinvar F3 Error Code

What does Lochinvar F3 mean?

The F3 fault on Lochinvar Shield water heaters signals that fuse F3 on the main control board has blown. Fuse F3 is a 1.25-amp slow-blow fuse that protects the main control board's internal logic circuits. A blown F3 fuse disables the control board's processing functions, causing the unit to lose all operational control and display capability.

Symptoms

• Control module display is blank with no indication of power.
• Unit does not initiate any heating cycle despite a call for heat.
• All indicator lights on the control board are off.
• Unit does not respond to any manual inputs at the control panel.

Common causes

• Transient voltage spike on the line-voltage supply causing the fuse to open.
• Internal fault on the main control board drawing excessive current through the fuse.
• Incorrect fuse installed previously (fast-blow instead of slow-blow) failing prematurely.
• Moisture or contamination on the control board creating a partial short through internal logic circuits.

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Diagnostic steps

1. Disconnect power and expose the control board

   Turn off power at the external disconnect switch. Remove the top access cover and control module cover to access the main control board.

2. Locate and test fuse F3

   Find fuse F3 on the main control board — it is the 1.25-amp slow-blow fuse. Use a multimeter set to continuity to confirm the fuse is open.

3. Replace fuse F3 with the exact specified rating

   Install a new 1.25-amp slow-blow fuse. Use only the fuse type specified on page 62 of the service manual. Do not exceed the rated amperage or substitute a fast-blow fuse.

4. Inspect the control board for moisture or contamination

   Before restoring power, examine the control board surface for signs of moisture, condensate, or corrosion near the fuse circuit. Allow any moisture to dry completely and clean lightly corroded contacts with electronic contact cleaner.

5. Restore power and monitor for recurrence

   Reinstall covers, restore power, and verify that the display activates and the unit fires normally. If the fuse blows again, the control board itself requires replacement.

When to call a professional

A control board that repeatedly blows F3 has an internal component failure that cannot be repaired in the field. Control board replacement involves reprogramming unit parameters and verifying combustion performance after installation. A licensed technician should perform control board replacement and post-replacement combustion analysis.