If your diesel air heater will not start or is not running properly, follow the checks below. Many problems can be fixed without sending the heater away for servicing.
This guide is written for Dieselheat, Belief and Eberspacher air heaters and hot water systems. The basic checks may also help with other brands, but fault codes, reset procedures and installation requirements vary by model.
You can follow this guide or watch our fault-finding video.
Getting Started
First, check the temperature setting. If your heater uses a thermostat, set the target temperature above the current room temperature or the heater will not try and start. Air heaters will also not start if the ambient temperature is over 35 degrees.
If you can, the first step to resolve your issue is to check fault codes. Please find your controller on the Downloads by Controller page on the Dieselheat website and use the instructions to retrieve any fault codes. We strongly recommend to retrieve the codes, clear them and then attempt to re-start the heater – then retrieve the codes again to see which ones are current.
Now – choose the description that best matches your heater:
| What is happening? | Where to start |
|---|---|
| The controller is blank or unresponsive, and nothing happens when you press a button. | Section A — No power |
| The heater blows cold air and tries to start for several minutes, but never produces heat. It then shows a failed-start fault code. | Section B — Tries to start but will not ignite |
| You have a current fault code: A fault (flashing LED), error code or fault alert appears almost immediately. The heater may barely turn, or briefly start and stop. | Section C — Fault before starting |
| The heater starts and produces heat, then stops and shows a fault. | Section D — Fault while running |
| The heater runs slowly, or repeatedly stops and restarts before the room reaches the set temperature. | Section E — Low output or overheating |
| The heater produces smoke or a diesel smell. | Section F — Smoke or diesel smell |
Section A – Dead control/heater
If the controller is blank start with the power supply.
1. Check the fuses
Find the heater fuses, usually near the battery or heater. Remove and test them, or replace them with fuses of the same rating. Test each fuse with a multimeter, or replace it with a known good fuse of the same rating. Visual inspection alone is not reliable—a fuse can look intact but still be faulty.
Typical arrangements covered by this guide are:
- DH Air 2.2kW: one 20 A heater fuse.
- Belief 2kW, 2.2kW, 4kW: one 20 A heater fuse.
- Eberspacher Air: a 20 A heater fuse and a separate 5 A controller fuse.
- Eberspacher Air, Water Combo: a 20A heater fuse and a 10A thermostat/controller fuse.
If a replacement fuse blows, contact Dieselheat.
2. Check for power at the heater
If you are comfortable using a multimeter, measure the supply voltage at the heater’s main power connector. Identify the correct pins using and avoid shorting them together.
- DH Air 2.2kW: Thick red (+) and black wires.
- Belief 2kW, 2.2kW, 4kW: Thick brown (+) and black wires.
- Eberspacher: Thick red (+) and brown wires.
Check both the main heater plug and the controller connection (where the controller connects to the heater loom).
No voltage at heater or controller: check the wiring and connections between the battery and heater.
Power at the heater and controller, but the controller is still dead: If power reaches both the heater and controller but neither responds, contact Dieselheat. The controller or the heater’s electronic control unit may need replacement.
3. Check the wiring and connections
Follow the wiring from the battery to the heater and controller. Look for damaged wires, loose connections or plugs that have come apart. Rough roads or shifting cargo can loosen connections.
Make sure all plugs are fully connected and all terminals are tight. Gently wiggle each connection while checking whether power returns to the controller.
Check for pins in connectors which are loose or have been pushed back.
Use a multimeter to check which sections of the wiring loom have power and which do not. Use the sharp tips of the probes to carefully pierce the power-wire insulation and test at different points. This can help locate where the power supply is interrupted.
Section B — Tries to start but will not ignite
The heater turns on and blows cold air, but does not produce heat. After several minutes it stops and reports a failed start.
No start fault codes include:
- Belief digital controller: E10.
- Belief rotary controller: continuous slow red flashing with no pauses.
- Eberspacher D2: error 52 or 53.
- Eberspacher D2L: a “Fuel Supply or Pump” message.
- Eberspacher Air, Water Combo: Alert symbol and error 12a, 12b or 125-129.
The main things to check are power, fuel, and the combustion air intake and exhaust.
Which check should I do first?
| When does the problem happen? | Check first |
|---|---|
| The heater struggles in cold weather, in the morning, or usually needs several attempts. | Power, then fuel |
| The heater previously worked reliably and has suddenly stopped starting or has been in storage. | Fuel |
| The heater has been stored, used in muddy or dusty conditions, or exposed to mud wasps. | Air intake and exhaust and fuel |
| The heater is newly installed or has run out of fuel. | Fuel supply and priming |
B1 Power — check the voltage while starting
A heater can have enough power to ‘kick over’ but not enough to heat the glow plug and ignite the fuel.
- Check battery voltage. Check the voltage via a monitor or multimeter to ensure it isn’t flat. Anything under 12.5 volts should be charged.
- Charge the battery. If possible, connect a suitable battery charger to help rule out a low battery.
- Check the power wiring. The heater should have the power supply, cable size and fuse protection specified for its installation. Loose connections, undersized cables and extra switches can cause voltage drop.
- Measure voltage at the heater during startup. A reading taken while the heater is off will not show the voltage drop caused by the glow plug.
Use a multimeter to measure the voltage at the heater while it is trying to start. During startup, the heater draws around 6–10 A as the glow plug heats up. This load can cause the battery voltage to drop and create voltage drop along the wiring. For the 12V installations covered by this guide, aim for approximately 12.4V or more at the heater during startup when troubleshooting. The actual operating limits depend on the heater model, ambient temperature, heater age etc.
Access for testing can be tricky. You may be able to insert the multimeter probes into the back of the heater’s main connector plug. If this is not possible, use the sharp probe tips to carefully pierce the power-wire insulation close to the heater.
If battery voltage remains healthy but voltage at the heater drops noticeably, check the cables and connections (see section A3). Repair any problems, then try starting again.
If the power supply is satisfactory, check fuel.
B2 Fuel — check that diesel reaches the heater
If the heater is smoking during start-up but not starting, it is likely that it is receiving fuel. A lack of smoke generally indicates no fuel is reaching the heater.
1. Check the tank
Make sure there is enough diesel in the tank. If the heater draws from a vehicle’s main tank, keep it above one-quarter full while testing: the heater pickup may not reach the bottom.
2. If the heater is new or has run out of fuel
Air in an empty fuel line must be cleared before diesel can reach the heater.
Use the priming or restart procedure for your model. Allow each start attempt and shutdown cycle to finish before trying again.
Do not keep restarting indefinitely. Repeated failed starts can cause a lockout or allow unburnt fuel to accumulate. Generally if the heater has not primed the line and started after 6-8 start cycles, further investigations in needed.
3. Check the fuel
Fuel which is older than 6 months should be checked and/or replaced.
Water can enter fuel tanks through a leaking cap, contaminated fuel or condensation. Collect a fuel sample in a clean, clear container and let it settle. Look for cloudy fuel, droplets or a separate layer of water at the bottom. Diesel floats above water.
If water or other contamination is present, clean the fuel system, refill with clean diesel and clear contaminated fuel from the lines by gravity draining or restarting the heater up to 6-8 times.
4. Check fuel delivery
If the heater is not smoking during startup, or you suspect fuel is not being delivered and you are comfortable disconnecting a fuel line:
- Switch the heater off and wait for shutdown to finish.
- Disconnect the fuel line at the heater and place the end in a suitable fuel-safe container, away from hot surfaces and ignition sources.
- Start the heater and check whether fuel is delivered as the pump pulses. Each pulse should spurt about a quarter of a teaspoon of fuel out of the pump.
- Switch off and allow shutdown to finish before reconnecting the line. Clean up any spills and check the connection for leaks.
No fuel delivered? Follow “No fuel flow” below.
Fuel delivered? Check for water, contamination or old fuel, or go to air intake and exhaust.
5. No fuel delivery – check the supply and pump
Fuel pumps are specific to each heater. Only replace the pump with one that exactly matches the original.
During the fuel-delivery stage of startup, the pump should tick or pulse. Newer pumps can be very quiet, so you may need to feel for the pulses.
If it does not pulse, re-check for a fault code. If the pump is not pulsing but there is no fault code then the pump may be mechanically jammed, in this case contact Dieselheat.
If the pump is ticking or pulsing but no fuel is being delivred:
- Check the pump installation. Confirm that the pump position (closer then 2m from the fuel source) and angle – approx. 15degrees outlet up.
- Check the fuel level and pickup. Make sure the pickup is submerged.
- Check the fuel condition. Contaminated fuel can block the pickup, lines, filter or pump.
- Check inlet-side connections. Even a small air leak between the tank and pump can stop the pump drawing fuel. Inspect fuel filters for cracks and all joiners for loose clamps.
- Check the pump’s inlet strainer, if fitted (Eberspacher and Belief). Watch this video on how to clean the inlet strainer.
If steps 1–5 above do not reveal a fault, connect a short length of fuel line to the pump inlet. Place the other end in a small, clean container of diesel close to the pump. Turn on the heater and check whether the pump draws fuel through the line and pumps it towards the heater.
If it does not pump fuel: The pump is likely faulty or blocked and will need to be replaced.
If it pumps fuel: Recheck steps 1–4, as this indicates an air leak or blockage in the fuel supply line before the pump.
If these checks do not restore flow, contact Dieselheat.
B3 Air intake and exhaust — check for blockages
The heater needs a clear combustion air intake and exhaust to burn fuel properly. These are separate from the ducts that circulate warm air through the living space.
Symptoms of partially blocked pipes include heater smoking but not starting or starting and continually smoking.
If the heater’s combustion air inlet and exhaust connections are outside, temporarily disconnect both pipes and try starting the heater. If it starts and runs properly with the pipes removed, but struggles with them connected, this indicates an obstruction in the intake or exhaust pipe.
With the heater switched off and cool:
- Inspect the combustion air intake and exhaust openings.
- Look for mud, insect nests, debris, crushed pipes or heavy carbon buildup.
- If necessary, remove the pipes for inspection. Mud wasps can build nests well upstream of the muffler.
- Clear obstructions with compressed ari or running water or replace damaged pipes as appropriate.
- Refit and secure the intake and exhaust before restarting.
Still will not start?
If the heater has a suitable power supply, clean fuel reaching it, and clear intake and exhaust pipes then contact Dieselheat to discuss servicing.
ll Dieselheat and talk to us about posting it back for a service. See the section on servicing here for instructions.
Section C — Fault before starting
If a fault code appears almost immediately (and the heater does not attempt to start), record the exact code before resetting anything.
- DH Air 2.2kW: note the “E” number.
- Belief digital controller: note the “E” number.
- Belief rotary controller: count the flashes between pauses. Use the fault-code guide for your controller.
- Eberspacher Air: follow the fault-retrieval instructions for your specific controller.
- Eberspacher Air, Water Combo: Alert symbol, follow the fault-retrieval instructions for your specific controller.
See this page for fault retrieval information by controller.
Search the Dieselheat documents and downloads for fault codes for all common heaters.
An immediate fault can indicate a an internal component problem. Look up the fault codes, if the fault remains after the recommended checks, contact Dieselheat with the heater model, controller type and fault code.
Section D — Fault while running
If the heater produces heat and then stops:
- Record the fault code if present. See this page for fault retrieval information by controller.
- Check fuel supply (Section B2), particularly for flame out or ignition failure faults.
For an overheating fault, go to Section E.
If it will not attempt to restart and produces an immediate fault code – go to Section C
If fault codes indicate flame out or ignition failure and it attempts to restart but will not start – go to Section B
Section E — Low output or overheating
For an air heater, first, check whether this is normal thermostat operation
A heater may slow down, switch off or restart automatically as the room reaches the set temperature.
If the room is still below the set temperature and the heater will not increase its output, check airflow and the temperature of the air entering the heater.
1. Check warm-air circulation
Look for anything restricting the flow of air through the heater:
- Closed or obstructed warm-air outlets.
- Crushed, blocked or excessively long ducting.
- Dust or pet hair blocking the heating-air inlet or fan.
- Insufficient ventilation into the cupboard or cavity containing the heater.
- Loose or disconnected ducting allowing hot air to return directly to the heater inlet.
Clear accessible obstructions and check that the ducting is attached securely. Contact Dieselheat if the fan needs internal cleaning or the duct layout needs assessment.
2. Check for hot air around the heater inlet
The room may be cool while the cupboard containing the heater is hot. Uninsulated warm-air ducting or leaking duct connections can heat the air entering the heater, causing it to reduce output or shut down.
Open the cupboard or access panel, keeping clear of hot and moving parts, and see whether operation improves.
If it does, the installation may need better ventilation, insulated ducting or a heating-air inlet duct that draws cooler air from the living space.
The temperature at which output reduces or the heater shuts down depends on the model and sensor arrangement.
Section F — Smoke or diesel smell
If smoke or exhaust fumes enter the living space, switch the heater off, ventilate the area and have the installation checked before using it again.
For smoke from the external exhaust, use the checks below.
Smoke after a difficult start
Unburnt fuel can cause temporary exhaust smoke after a difficult start. It can persist for quite a while, but should reduce as the heater warms up and unburnt fuel in the exhaust vaporises.
If the smoke is heavy, continues, or is accompanied by fuel leakage or abnormal operation, switch off and refer to Section B3 air intake and exhaust.
Smoke during normal operation
- Check the combustion air intake and exhaust for partial blockages, as described in Section B3.
- Check the fuel for water or contamination.
- If the heater has spent long periods running at low output, it may have carbon buildup.
If the installation is sound, there are no leaks or blockages, and the heater otherwise runs normally, running at full power may help clear light deposits. If it still smokes after 15–20 minutes, contact Dieselheat about servicing.
Need help or a service?
If these checks do not resolve the problem, contact Dieselheat.
Please have the following ready:
- Heater brand and model.
- Controller type.
- Exact fault code or flashing pattern.
- What happens when you try to start it.
- Checks you have completed and any voltage readings taken during startup.
Contact us before sending your heater in. We can help confirm whether the problem can be solved locally or requres service by us. Read our diesel heater servicing information.
Fault codes and controller instructions
Use the fault-code list and operating instructions that match your heater and controller.
