Furnace shuts down with flue gas sensor error, may run briefly then lock out

Safety: Turn off power and ensure the flue is cool before touching the flue gas sensor or piping.

• Inspect flue gas sensor: Locate the flue gas NTC sensor on the Viessmann flue collector or exhaust manifold. Check that it is firmly clipped in place and that the cable is not damaged.
• Check wiring: Follow the sensor cable back to the control board. Reseat the plug and look for corrosion or moisture on the connector.
• Check for flue blockage: From outside, inspect the terminal for obstructions (ice, debris, nests). A blocked flue can cause high flue temperatures and trigger F5.
• Reset and test: Restore power and reset the boiler. If the code returns immediately, the sensor may be open or shorted and should be replaced.
• Professional testing: A technician can measure the sensor resistance at room temperature and compare it to Viessmann specifications to confirm failure.