Short answer: E27 on an HLN air heater (Aerolyn 2kW / 4kW / 8kW) means overheating detected at the overheat sensor. On 8kW rotary-switch models the same fault is flashed as blink code 11011. Nine times out of ten this is an airflow problem — the heat is being made faster than the ducting can carry it away.

What E27 means

The overheat sensor sits on the heat exchanger. When its temperature crosses the safety threshold, the ECU shuts the heater down and raises E27. Unlike E05/E06 (sensor circuit faults), E27 means the sensor is working and genuinely measured too much heat.

Factory diagnosis

1. Check the air ducting for excessive restriction or blockage — crushed duct, a closed or blocked outlet, clogged intake screen.
2. Check the ducting length against specification (refer to the product catalogue) — runs that are too long or have too many bends back the heat up into the heater.
3. Measure the resistance of both the overheat sensor and the flame sensor to confirm they are within specification.
4. Perform the fuel quantity test — overfuelling makes more heat than the airflow can remove.

FAQ

The heater works with the duct off but E27 with it on — what does that mean?
That is the ducting telling you it is the restriction. Shorten the run, remove bends, increase the duct diameter to spec, and make sure every outlet is open.

Can I just keep restarting it?
The heater will keep protecting itself. Repeated overheat cycles stress the heat exchanger — fix the airflow first.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E27 is 11011 — two long, one short, two long (long = 1, short = 0).

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