Error Code	Symptom	Fault Type	Guide
E01	Controller displays E01 and the furnace does not start the ignition sequence	Ignition Failure – No Flame Detected at Start	View Details
E02	E02 displayed and furnace shuts down shortly after ignition attempt	Flame Loss During Operation	View Details
E03	E03 error and furnace stops; display may indicate overheat or high boiler temperature	Overheat Protection Activated	View Details
E04	E04 displayed; furnace does not start fan or ignition sequence	Exhaust Fan / Draft Fan Failure	View Details
E05	E05 error and furnace stops; controller may show flue temperature fault	Flue Gas Temperature Sensor Fault	View Details
E06	E06 displayed; furnace does not respond to temperature changes or shows unrealistic boiler temperature	Boiler Temperature Sensor Error	View Details
E07	E07 error; controller indicates hopper or fuel temperature alarm	Fuel Hopper Temperature Overlimit	View Details
E08	E08 displayed; controller indicates feeder or auger fault and fuel is not being delivered	Screw Feeder (Auger) Jam or Motor Fault	View Details
E09	E09 error; controller indicates low water pressure or lack of water in system	Low System Pressure / Water Shortage	View Details
E10	E10 displayed; controller indicates circulation pump fault or no flow	Heating Circuit Pump Failure	View Details
E11	E11 error; controller indicates return temperature sensor fault	Return Water Temperature Sensor Error	View Details
E12	E12 displayed; controller indicates room thermostat communication or sensor error	Room Thermostat / Room Sensor Fault	View Details
E13	E13 error; controller indicates lambda sensor or O2 regulation fault (on Defro models with lambda control)	Lambda (Oxygen) Sensor Fault	View Details
E14	E14 displayed; controller indicates fan speed error or tachometer fault	Combustion Fan Speed Feedback Error	View Details
E15	E15 error; controller indicates door or hatch open	Door / Service Hatch Switch Open	View Details
E16	E16 displayed; controller indicates power supply or internal electronics error	Control Board Power Supply Fault	View Details
E17	E17 error; controller indicates buffer tank sensor fault (for systems with buffer/accumulator tank)	Buffer Tank Temperature Sensor Error	View Details
E18	E18 displayed; controller indicates DHW (domestic hot water) sensor fault	DHW Cylinder Temperature Sensor Error	View Details
E19	E19 error; controller indicates communication fault with expansion module or remote panel	Communication Bus Error	View Details
E20	E20 displayed; controller indicates pellet/coal out or no flame due to lack of fuel	Fuel Depletion / No Fuel Supply	View Details
–	Ash pan fills very quickly and ash is unusually coarse or contains unburned fuel	Inefficient Combustion / Incomplete Fuel Burn	View Details
–	Boiler pressure rises excessively during heating and safety valve discharges water	Expansion Vessel / Safety Valve Issue	View Details
–	Buttons on the Defro controller do not respond or respond intermittently	Keypad / Control Panel Fault	View Details
–	Condensation or water dripping from flue connection and back of furnace	Condensation in Flue / Low Return Temperature Corrosion Risk	View Details
–	Controller display is blank or unreadable, but power is present at the unit	Display / User Interface Failure	View Details
–	Frequent tripping of the main fuse or circuit breaker when furnace starts	Electrical Overload / Short Circuit	View Details
–	Furnace casing or control panel becomes excessively hot to the touch during operation	Overheating of Casing / Insulation Degradation	View Details
–	Furnace frequently shuts down and restarts, with large temperature swings	Short Cycling / Poor Control Tuning	View Details
–	Furnace makes loud banging or knocking noises in the pipes when heating	Water Hammer / Boiling in Heat Exchanger	View Details
–	Furnace runs but radiators remain lukewarm or cold, especially on upper floors	Insufficient Heat Output / Poor Circulation	View Details
–	Room thermostat calls for heat but furnace does not start; display shows standby with no error code	Control Logic / External Control Inhibit	View Details
–	Strong smoke or soot deposits visible at chimney outlet and on nearby surfaces	Incomplete Combustion / Sooting	View Details
–	Strong tar or creosote smell around furnace and in boiler room	Low‑Temperature Operation / Tar Formation	View Details
–	Unusual grinding or squealing noise from the screw feeder area during operation	Mechanical Wear or Obstruction in Screw Feeder	View Details
–	Visible smoke leakage from furnace doors or joints into the boiler room	Flue Gas Leakage / Poor Chimney Draft	View Details
–	Water leaking from under the furnace or from boiler casing joints	Boiler Water Leak	View Details

Defro Furnaces Error Codes

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