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E03 (blink code 00011) means supply voltage has been too low for at least 20 seconds. Limits: above 10V on 12V systems; above 21V (air) / 20V (coolant) on 24V systems. Check battery, fuses, wiring voltage drop and the charging system.

E02 (blink code 00010) means supply voltage is too high. Limits: air heaters below 16V (12V systems); coolant heaters below 18V (12V systems); both below 32V on 24V systems. Disconnect any charger and check the vehicle charging system.

E01 (blink code 00001) means a short circuit in one of the heater's electrical loads — blower motor, pump, glow pin or ECU. Check each load's wiring in turn; the ECU is the last suspect.

E00 (blink code 00000) means the flame sensor circuit is open — its resistance reads too high. Check the sensor resistance first; replace the flame sensor, and the ECU only if the fault persists.

E21 (blink code 10101) means excessive current on the blower motor circuit. One current measurement tells you whether to replace the motor or look at the ECU — do the test before buying parts.

E13 (blink code 01101) means combustion did not establish during the start phase. Work through the factory checklist in order: airflow, fuel grade, glow pin screen, pump pulsing, fuel quantity.