Furnace shows E05 and does not start; may occur immediately on power-up.

Safety: Isolate power before accessing the control compartment.

• Locate the flow NTC: On Chaffoteaux furnaces, the flow temperature sensor is usually clipped to the flow pipe or inserted into a pocket on the primary heat exchanger.
• Check connections: Ensure the NTC plug is fully inserted into the wiring harness and that the harness is firmly connected to the main control board.
• Inspect for damage: Look for signs of water ingress, corrosion, or burnt connectors around the sensor and wiring. Any signs of overheating or melting indicate the need for professional replacement.
• Basic resistance check (for professionals only): With power off and sensor disconnected, a technician can measure the NTC resistance with a multimeter and compare it to the Chaffoteaux service chart for the current water temperature.
• Reset: After reseating connectors, restore power and reset the furnace. If E05 persists, the NTC sensor or control board must be replaced by a qualified technician.