No hot water at any faucet; Navien tankless water heater display is on but burner does not fire and no error code is shown.

Safety: Water-side checks are generally safe, but turn OFF power if removing covers or sensors.

Steps:

• 1. Check multiple fixtures: Open more than one hot water tap to increase flow. If the burner fires only at higher flow, the flow sensor may be marginal or filters clogged.
• 2. Verify valves: Ensure the cold water inlet valve and any service valves at the Navien are fully open.
• 3. Clean inlet filter: Remove and rinse the cold water inlet filter screen to clear sediment.
• 4. Check shower mixing valves: Anti-scald or thermostatic valves can reduce flow; test at a simple faucet (e.g., laundry sink) to rule out fixture issues.

If the unit still does not ignite with adequate flow and no error code, the flow sensor may be failing and should be tested or replaced by a technician.