This is the official HLN error-code reference, compiled directly from our factory service manuals. Important: error-code numbering is brand-specific. An E08 on an HLN heater does not mean the same thing as an E08 on a Webasto, Espar, or generic-brand heater — this table applies to HLN heaters only. The E-code table below is used by HLN hydronic (Aquano) heaters and HLN digital controllers; Aerolyn air heaters with the standard indicator light use the 5-pulse flash codes listed near the end of this guide. Find your code, follow the steps in order, and you will resolve most faults without tools beyond a screwdriver and a multimeter.

⚠️ Safety first: fuel and exhaust work must be done with the heater off and cooled down. Exhaust must always terminate outside the vehicle. If in doubt, ask us before improvising.

E01 – Short Circuit (Blower, Water Pump, FMP, or ECU)

E01 indicates a short circuit in the blower motor, water pump, fuel metering pump, or ECU. Steps: check blower motor, water pump, and glow pin for short circuit; check fuel metering pump and wiring for short circuit; check battery connections for corrosion and proper contact; test ECU and replace if necessary.

E02 – Overvoltage Fault

E02 indicates overvoltage. For a 12V heater, voltage must be below 18V. For a 24V heater, voltage must be below 32V. Disconnect any battery charger if connected. Check the vehicle charging system and correct if necessary.

E03 – Undervoltage Fault

E03 indicates undervoltage detected for 20 or more seconds. For a 12V heater, voltage must be above 10V. For a 24V heater, voltage must be above 20V. The wiring harness voltage drop must not exceed 1V. Check fuses, connections, and wiring for damage; check battery connections for corrosion; check the vehicle charging system.

E08 – Fuel Metering Pump Short Circuit

E08 indicates a short circuit in the fuel metering pump. Disconnect the FMP connector and restart the heater. If E08 persists, the FMP is defective. If E08 still displays after disconnecting the harness, look for a short circuit in the FMP wiring. If no short circuit is found, test the ECU and replace if necessary.

E11 / E12 – Water Temperature Sensor Fault

E11 is a short circuit and E12 is an open circuit on the water temperature sensor. For both: remove the control unit, unplug the sensor, and test resistance. If resistance is greater than 2000 kΩ or less than 3 kΩ, the sensor is damaged and must be replaced. Test the ECU and replace if necessary.

E13 / Fault 13 – No Flame Detected

E13 means no flame was detected during the start phase. The blower runs, the fuel metering pump starts, then the heater shuts down after two attempts. Diagnostic steps: check glow pin resistance at room temperature (12V heater: 0.4–0.6 Ω; 24V heater: 1.2–2.0 Ω); check combustion air intake and exhaust lines for obstruction; verify fuel grade is appropriate for ambient temperature; check vent hole for clogging; confirm fuel metering pump is receiving pulse signal and check wiring harness; perform Fuel Quantity Test (Section 4); inspect glow pin for carbon deposits — clean or replace.

E14 / Fault 14 – Combustion Interruption After Start

E14 means the flame was extinguished after a successful start. Diagnostic steps: check fuel level in tank; perform Fuel Quantity Test (Section 4); inspect fuel lines for air bubble formation; check combustion air intake and exhaust; verify correct fuel grade for temperature; check glow pin resistance and mounting; check water pump, coolant level, and circulation pipe valve; check water pipe for restriction or blockage; test ECU and replace if necessary.

E18 / E19 – Glow Pin Open or Short Circuit

E18 is an open circuit and E19 is a short circuit on the glow pin. Expected resistance at 20°C: 12V heater — 0.4 to 0.6 Ω; 24V heater — 1.2 to 2.0 Ω. Check the glow pin harness for damage, routing, and connection. Check harness for continuity. Test ECU and replace if necessary.

E21 – Blower Motor Short Circuit

E21 indicates a short circuit in the blower motor. Check wiring for short circuit. For a 12V heater: apply 12V to the blower — if current is less than 2.0 A, test/replace the ECU; if greater than 2.0 A, replace the blower. For a 24V heater: apply 24V — if current is less than 1.1 A, test/replace the ECU; if greater than 1.1 A, replace the blower. The power supply must have at least 20 A short circuit resistance.

E22 – Water Pump Short Circuit

E22 indicates a short circuit in the water pump. Check wiring for short circuit. For a 12V heater: apply 12V — in the first 5 minutes (35W draw), if current is less than 2.9 A, test/replace the ECU; if greater than 2.9 A, replace the pump. For a 24V heater: apply 24V — if current is less than 1.46 A, test/replace the ECU; if greater than 1.46 A, replace the pump. The ECU triggers this fault when pump current exceeds 8 A (12V) or 5 A (24V). After 5 minutes, pump power reduces to 18W.

E25 – Altitude Sensor Fault

E25 indicates an altitude sensor fault. Test the ECU and replace if necessary.

E30 / Fault 30 – Overheating or Poor Water Circulation

E30 indicates dry burning or poor water circulation. Check if the water pump is running. Check if the circulation pipe valve is open. Inspect for restricted or blocked water pipes.

E32 – Digital Controller No Feedback Signal

E32 means the digital controller cannot detect a feedback signal. Steps: (1) Check that the wire harness is correctly connected to the digital controller. (2) Replace the digital controller. (3) Test ECU and replace if necessary.

Heater Does Not Start, Indicator Light Off

1. Check the fuse in the power harness. 2. Check power to the control unit. 3. Check power to and from the switch. 4. Inspect all electrical connections.

Heater Smokes and Carbons Up – Causes and Fixes

Possible causes and checks: blocked exhaust pipe; blocked combustion air intake; exhaust re-entering the combustion air intake pipe; insufficient supply voltage; circulation pipe valve closed or water pipes restricted/blocked; fuel system issue or incorrect fuel metering pump position or quantity; incorrect motor RPM.

No Fuel Metering Pump Activity – Blower Runs

Check for electrical pulses at the fuel metering pump. Check if the pump is frozen. Inspect for a blocked fuel line. Verify the pump is plugged in and the circuit is not open. Replace the pump if faulty.

Heater Does Nothing When Turned On

1. Check voltage on the heater harness while the heater is switched ON. 2. If voltage is OK, connect the red and yellow wires on the control branch together to bypass the switch and confirm +12V or +24V on the red wire. 3. If the heater still does not start, the ECU is likely faulty.

Two Start Attempts, No Exhaust Smoke, No Ignition

1. Check if the exhaust pipe is blocked with ice; reroute if U-shaped. 2. Check the combustion air intake pipe for obstruction. 3. If pipes are clear, check that the motor runs normally. 4. If motor and pipes are OK, proceed to fuel quantity testing.

Heater Smokes Briefly Then Stops After Two Attempts

1. Perform the Fuel Quantity Test. If fuel is insufficient, check the fuel pick-up pipe, lines, connections, and filter; replace filter or pump as needed. 2. If needed, disassemble the heater and clean or replace the glow pin. Clean vent holes, combustion tube airways, and heat exchanger. 3. If the combustion chamber has heavy carbon or ceramic-like buildup behind the ring wall, replace the flame tube.

Heater Ignites But Flame Not Detected, Then Stops

The heater ignited but the flame was not detected. First, check and replace the glow pin. If the glow pin measures within specification, replace the ECU.

Heater Starts But Frequently Stops With Fault Code

Check the following in order: (1) Fuel level in tank. (2) Fuel lines for gaps at connection points. (3) If the heater stops only when the vehicle is moving, reroute air intake and exhaust pipes with ends bent toward the rear of the vehicle. (4) Check the water pump. (5) Check coolant level and condition. (6) Confirm the circulation pipe valve is open. (7) Check for restriction or blockage in the water pipe. (8) Verify the water pipe length is within specification.

Power Indicator Light Not Turning On

Check the following: (1) Confirm the main power is turned on. (2) Check whether the heater fuse is blown. (3) Check whether the wire harness plug is connected firmly and making good contact.

Switch Status Indicator Lamp Flickering

A flickering status indicator lamp (outside of fault code flashes) indicates a possible power supply issue. Check: (1) Whether the power supply voltage is stable. (2) Whether the stabilised voltage supply capacity is sufficient. (3) Whether the control box is damaged.

Indicator Light On But Heater Does Not Start

Try starting again and check if the fuel line is filled (especially on first start). Then check: (1) Sufficient fuel in the tank. (2) Fuel line for air leakage. (3) Combustion air inlet and outlet for blockages. (4) Ignition plug for carbon deposition or burn-off. (5) Sensor and controller for damage.

Unstable Combustion

Check the vent for fuel dripping and smoking. Also check whether the fuel line is blocked or loose.

Automatic Cooling Cycle Stops Early

Check whether the air inlet and outlet are obstructed. Also check whether the heater air inlet temperature is too high.

Aerolyn Air Heaters: How to Read the Fault Flash Codes

Aerolyn air heaters with the standard indicator report faults as groups of 5 pulses. A slow flash (1 second on) represents binary 1; a fast flash (0.2 seconds on) represents binary 0. Pulses are separated by 0.5-second intervals. Each 5-pulse group repeats after a 3-second pause. For example, five fast flashes = fault code 00000.

Complete Aerolyn Flash Code List

00000: Combustion sensor broken. 00010: Power supply voltage too high. 00011: Power supply voltage too low. 00100: Combustion sensor short circuit (resistance too small). 00101: Overheat sensor broken. 00110: Overheat sensor short circuit. 01000: Self-checking current too high when electromagnetic pump starts. 01101: Misfire. 10011: Self-checking current too high when ignition plug starts. 10101: Self-checking current too high when main motor starts. 11011: Heater overheating (excessive air outlet temperature). 11101: Hall sensor cannot detect magnet (motor not turning, locked-rotor, or controller not properly positioned).

Heater Ignition Retry and Fault Code Behavior

If the heater fails to ignite, it will attempt a second ignition. If it still fails, or if the control box cannot detect the feedback signal during normal operation, the heater will stop igniting. After 90 seconds of cooling down, the status indicator will display a fault code. The cause of abnormal operation should then be diagnosed.

Common replacement parts: glow pin, fuel pump, WiFi controller.

About HLN — For 50+ years HLN has built diesel air and hydronic heaters as original equipment for major truck, bus and RV manufacturers (YuTong, SANY, XCMG and two Fortune-500 truck makers), with CE / ECE R122 / R10 certification. Shop the heaters this guide covers: Aerolyn 2kW, Aerolyn 4kW, Aquano 5K hydronic. Questions? Email kevin@hlnind.com — real engineers answer.