Lochinvar APS Error Code

What does Lochinvar APS mean?

The APS fault on Lochinvar Armor and Shield water heaters indicates that the air pressure switch contacts are open. The control module monitors this switch to confirm the combustion air blower is moving adequate airflow before allowing ignition. When the switch contacts fail to close, the unit locks out and will not attempt ignition until the condition is corrected.

Symptoms

• Unit fails to ignite and displays APS fault on the control module screen.
• Blower motor runs but burner never fires.
• Repeated lockout cycles that do not self-clear.
• Visible obstruction at the air intake termination cap.

Common causes

• Loose or disconnected wiring at the common and normally-closed terminals of the air pressure switch.
• Vent or air intake pipe lengths exceed the maximum allowed, creating excessive static pressure drop.
• Obstruction or blockage in the air intake pipe or at the termination fitting.
• Kinked, cracked, or water-logged reference hose between the switch and the blower housing.
• Fouled burner or scaled heat exchanger increasing combustion air resistance.
• Defective air pressure switch that will not close even with adequate airflow.

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Diagnostic steps

1. Inspect wiring at the air pressure switch

   Check the wiring connections at the common and normally-closed terminals on the switch. Reseat any loose spade connectors and inspect the wiring harness for damage or corrosion.

2. Verify vent and air intake pipe lengths

   Confirm that total vent and air intake pipe lengths comply with the specifications in Section 2 of the service manual for your model. Reduce pipe lengths or eliminate unnecessary elbows if the installation exceeds the maximum.

3. Clear blockages from intake pipes and termination

   Inspect the full length of the air intake pipe and the termination fitting for debris, bird nests, ice, or other obstructions. Clear any blockage and verify unobstructed airflow.

4. Check reference hoses

   Disconnect the reference hoses at the air pressure switch and the blower housing. Shake out any accumulated condensate, inspect for kinks or cracks, and reconnect securely.

5. Clean the burner and heat exchanger

   If airflow resistance is elevated due to combustion deposits, clean the burner assembly and heat exchanger per the cleaning procedure in the service manual (typically pages 60–61 on Shield models).

6. Test and replace the air pressure switch if defective

   With the blower running, use a manometer or continuity tester to verify that the switch closes at the specified differential pressure. Replace the switch if it does not close at the correct pressure differential.

When to call a professional

If the APS fault persists after clearing blockages, correcting pipe lengths, and replacing the switch, the combustion air blower itself or the main control board may be at fault. Combustion system diagnosis, gas valve interlock verification, and heat exchanger cleaning require proper tools and safety procedures. Call a licensed HVAC/R technician if you cannot confirm safe combustion conditions after completing the above steps.