Lochinvar water heater shows E004 and will not fire; fan may run continuously.

Safety: Turn off power and gas before working on venting. Hot flue gases can cause burns; allow the unit to cool.

Steps:

• 1. Inspect exhaust vent: From the heater to the termination, check for sagging PVC, standing condensate, or physical blockages. Correct any low spots and remove obstructions.
• 2. Verify vent design: Compare installed vent length, diameter, and number of elbows to the Lochinvar installation manual. Excessive length or elbows can cause high flue temperatures and lockouts.
• 3. Check terminations: Ensure intake and exhaust terminations are properly spaced and not recirculating exhaust into the intake. Clear any debris or snow around terminations.
• 4. Test flue temperature sensor: Locate the flue temperature sensor (often a thermistor mounted in the exhaust outlet). With power off, disconnect and measure resistance; compare to Lochinvar resistance/temperature chart. Replace if readings are out of range or sensor is physically damaged.
• 5. Reset and test: After corrections, restore power and reset the fault. Monitor operation and verify that exhaust temperatures remain within specification.