Short answer: E25 on an HLN diesel heater means an altitude (atmospheric-pressure) sensor fault — on coolant heaters, the pressure reading is implausibly low. The sensor is built inside the ECU, so the remedy is to test the ECU and replace it if necessary. On 8kW rotary-switch models the same fault is flashed as blink code 11001.

What E25 means

HLN heaters adjust their fuel-air mixture automatically with altitude (up to 16,000 ft / 5,000 m on the PRO air heaters). That compensation relies on the atmospheric-pressure sensor on the ECU board. When its reading goes implausible on the low side, the ECU raises E25.

Factory diagnosis

1. There is no external sensor to check — the pressure sensor is a component of the ECU board.
2. Test the ECU and replace it if necessary.

FAQ

I'm at high altitude and got E25 — is the altitude itself the cause?
No. Normal high altitude is exactly what the sensor is designed for. E25 means the sensor's reading is faulty, not that you are too high.

What's the difference between E25 and E26?
E25 = pressure reading too low; E26 = pressure reading too high. Both point at the same sensor inside the ECU, and the remedy is the same.

My 8kW heater has no screen — how do I see this code?
Count the status-light flashes: E25 is 11001 — two long, two short, one 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).