Short answer: E05 on an HLN air heater means the ECU has detected an open circuit on the overheat sensor — it records an open circuit when the resistance is greater than 2000 kΩ. On 8kW rotary-switch models without a screen, the same fault is flashed as blink code 00101.

What E05 means

The overheat sensor is the heater's thermal safety: it lets the ECU shut the heater down before the heat exchanger gets dangerously hot. When its circuit is open the ECU can no longer trust its overheat protection, so it raises E05 and stops. On air heaters the overheat sensor and the flame sensor are supplied together as a combo sensor.

Factory diagnosis (Aerolyn 2kW / 4kW / 8kW)

1. Disconnect both the flame and overheat sensors from the ECU.
2. Measure the resistance between the blue and brown/white wires and compare against the temperature–resistance indexing table in the service documentation.
3. If the resistance is within the allowable range, the sensor is fine — test the ECU and replace it if necessary.
4. If the resistance is out of range (open), replace the combo sensor.

FAQ

What's the difference between E05 and E06?
E05 is an open circuit (resistance too high); E06 is a short circuit (resistance too low). Same sensor, opposite electrical failure.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E05 is 00101 — two short, one long, one short, one long (long = 1, short = 0).

Is E05 the same on other heater brands?
No. Fault-code tables are brand-specific — this table applies to HLN heaters only.

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