Viessmann [E3] fault codewhat it means and how to fix it

E3 on a Vitodens is a flow temperature sensor fault: the NTC reporting the temperature of water leaving the heat exchanger has returned a value the burner control unit will not accept. Nearly every decision the appliance makes depends on that reading — how hard to fire, when to modulate, when to stop — so it shuts down rather than operating without it. In practice this is far more often a failed sensor or a wiring fault than a real overtemperature, which is worth knowing because it makes E3 one of the less alarming codes on the list despite how it sounds. Viessmann use a broad group of codes for sensor short circuits and open circuits across the range, and an engineer will measure the NTC resistance against the published curve rather than replacing it on the strength of the code. The alternative scenario, where the sensor is accurate and the water really is that hot, would normally be accompanied by an overheat code such as F5 or F2, so the absence of one is reassuring.