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E30 on the HLN Aquano coolant heater means dry burning or poor water circulation. Three checks: is the water pump running, is the circulation valve open, are the water pipes clear.

E29 (blink code 11101) on HLN air heaters means blower speed below 300 rpm. Check for blockage, a distorted mounting surface, and — a documented real-world cause — a power source that can't deliver the 15 A the heater needs at start.

E27 (blink code 11011) on HLN air heaters means overheating detected at the overheat sensor — almost always an airflow problem. Check ducting restriction and length, verify both sensors, then run the fuel quantity test.

E26 (blink code 11010) on HLN coolant heaters means the atmospheric-pressure sensor inside the ECU reads implausibly high. The only remedy is to replace the ECU.

E25 (blink code 11001) means an altitude / atmospheric-pressure sensor fault. The sensor lives inside the ECU, so the remedy is to test and replace the ECU — there is no separate sensor part to swap.

E22 (blink code 10110) on HLN coolant heaters means the water pump is drawing too much current — a stalled, shorted, damaged or debris-jammed pump. A simple current measurement tells you whether to replace the pump or the ECU.