Short answer: E01 on an HLN diesel heater means the ECU has detected a short circuit in one of its electrical loads — the current draw is too high. On 8kW rotary-switch models without a screen, the same fault is flashed as blink code 00001. Work through the loads one by one; the ECU itself is the last suspect.

What E01 means

The ECU powers several loads — the blower motor, the fuel metering pump, the glow pin, and on coolant heaters the water pump. When load current is too high, the ECU raises E01 and shuts down to protect itself and the wiring.

Factory diagnosis

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

1. Check if the blower motor has a short circuit.
2. Check if the fuel pump or its wiring has a short circuit.
3. Test the ECU and replace it if necessary.

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

1. Check the blower motor, water pump and glow pin for short circuits (check the current draw of each load).
2. Check the fuel metering pump and its wiring for a short circuit.
3. Check the battery connections for corrosion and proper contact.
4. Test the ECU and replace it if necessary.

FAQ

Which load is most often the culprit?
The diagnosis order above is the factory order — check the motor and pump wiring before suspecting the ECU. Chafed insulation and pinched wires where the harness passes through the floor or firewall are common causes.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E01 is 00001 — four short flashes then one long (long = 1, short = 0).

Is E01 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).