Short answer: E00 on an HLN diesel heater means the ECU has detected an open circuit on the flame sensor — the sensor's resistance reads too high. On 8kW rotary-switch models without a screen, the same fault is flashed as blink code 00000. The fix is usually a flame sensor replacement; the ECU is only replaced if the fault remains afterwards.

What E00 means

The flame sensor tells the ECU whether combustion is actually happening. When the circuit is open — a broken wire, a corroded connector, or a failed sensor element — the ECU cannot read it and raises E00. On coolant heaters the sensor is a platinum-resistance element.

Factory diagnosis

Air heaters (Aerolyn 2kW / 4kW / 8kW)

1. Disconnect the flame sensor (the green connector) and measure its resistance against the temperature–resistance specification table in the service documentation.
2. If the resistance is within specification, the sensor is fine — test the ECU and replace it if necessary.
3. If the resistance is out of range (or open), replace the sensor.

Coolant heaters (Aquano 5K / 10K / 12K / 15K)

1. Replace the platinum-resistance flame sensor.
2. If E00 persists after the sensor is replaced, replace the ECU.

FAQ

Is the flame sensor the same part as the overheat sensor?
On air heaters they are supplied together as a combo sensor. Open/short faults on the overheat side raise E05/E06 instead — the code tells you which side of the sensor is affected.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E00 is 00000 — five short flashes (long = 1, short = 0).

Is E00 the same on other heater brands?
No. Fault-code tables are brand-specific — this table applies to HLN heaters only. Webasto, Espar and other brands number their faults differently.

All HLN fault codes in one place: HLN Diesel Heater Error Codes Explained (E01–E32). Source: HLN factory service manual (2026 revised editions).