Lochinvar F2 Error Code

What does Lochinvar F2 mean?

The F2 fault on Lochinvar Shield water heaters indicates that fuse F2 on the main control board has blown. Fuse F2 is a 3.15-amp slow-blow fuse that protects the 24-volt control circuit supply. When this fuse opens, the 24V control circuit loses power and the unit cannot operate or display normal status.

Symptoms

• Unit is completely non-responsive with no display or operation.
• 24-volt control circuit devices such as the gas valve and zone controls do not energize.
• Unit does not respond to thermostat or aquastat calls for heat.
• No error message displayed because the control board itself has lost 24V power.

Common causes

• Short circuit in the 24-volt wiring harness or a connected control device drawing excessive current.
• Failed transformer putting excessive voltage or current through the 24V supply.
• Damaged or pinched wiring on the 24V circuit causing an intermittent or sustained short.
• Fuse weakened from repeated thermal cycling and blown on normal demand.

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Diagnostic steps

1. Turn off power and access the control board

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

2. Locate and inspect fuse F2

   Identify fuse F2 on the control board — it is a 3.15-amp slow-blow fuse protecting the 24V supply circuit. Use a multimeter on continuity mode to verify the fuse is open.

3. Replace fuse F2 with the correct rating

   Install a new 3.15-amp slow-blow fuse of the exact amperage specified. Do not substitute a fast-blow fuse or jumper the fuse holder. Spare fuses are typically included in a plastic bag attached to the cover of the unit.

4. Inspect the 24V wiring for shorts before restoring power

   Before replacing the fuse, inspect the 24V wiring harness for pinched wires, damaged insulation, or shorted connectors. Repair any damage found to prevent the new fuse from blowing immediately.

5. Restore power and verify operation

   Reinstall covers, restore power, and verify that the control module display comes on and the unit responds normally to a call for heat.

When to call a professional

If fuse F2 blows again immediately after replacement, a sustained short circuit exists in the 24V wiring or a connected control component. Tracing intermittent shorts in low-voltage circuits requires a clamp meter and systematic isolation of each branch circuit. Contact a licensed technician to diagnose the root cause before additional fuses are damaged.