Display shows E05, burner does not start, fan may run briefly

Safety: Isolate power before removing covers or disconnecting sensors.

• 1. Power cycle: Turn off power for 30 seconds and restart. If E05 returns immediately, proceed with sensor checks.
• 2. Locate flow NTC: The ACV flow temperature sensor is usually clipped or screwed into a pocket on the flow pipe or heat exchanger outlet. Consult the unit's schematic for exact position.
• 3. Inspect wiring: Check the sensor cable for cuts, burns, or crushed sections. Ensure the plug is fully seated on the control board and at the sensor.
• 4. Measure resistance: With power off and wires disconnected, measure the NTC resistance with a multimeter. Compare to ACV's temperature/resistance chart. An open circuit or near-zero resistance indicates a faulty sensor.
• 5. Replace sensor: If readings are out of range, replace the ACV flow NTC sensor with the correct part number. Use thermal paste if specified and ensure good contact in the sensor pocket.

If the new sensor does not clear E05, the main control board may be faulty and should be tested by a professional.