Diesel Air Heater Fault Finding

Diesel Air Heater Fault Finding

If your diesel heater will not start, please follow the steps in this document carefully. In most cases you will be able to fix it yourself, saving you time and money. The team at Dieselheat can also help you if following these steps doesn’t work.

This fault finding document is set up for the brands we sell, Eberspacher and Belief – but is also suitable for other heater brands as a general method to troubleshoot a heater that will not start.

Getting Started

See the following descriptions to determine how you should get started with re-starting your diesel heater.

  1. If the heater or control is dead and nothing happens when any button is pressed, go to Section A.
  2. If the heater turns on, blows cold air and tries to start over a period of about 6 minutes, but does not start and then shows either E10 (Belief digital controller) or the red LED flashes continuously with no pauses (Belief rotary controller) or Error 53 Eberspacher D2, go to section B.
  3. If the heater turns on but the fan hardly spins, or spins briefly and stops, and then an error code is shown (digital control) or the red LED flashes an error code (rotary controller), go to Section C. (An error code on the digital controller will be “E” and a number. On the rotary controller, the red LED will flash between 1 and 8 times with a pause between groups of flashes).
  4. If the heater was running and then stopped and displayed an error code (digital control) or the red LED flashes (rotary controller), go to section D.
  5. If the heater is running but will not speed up, just running slowly all the time regardless of the power setting, go to Section E.
  6. If the heater is running but is making a smoke or diesel smell, go to section F.

NOTE if you have an Eberspacher heater or any other brand of heater that operates on a thermostat and you do not set the target room temperate above the actual room temperature then the heater will not start. Heaters will also not start if the room is above approx 35 degrees.

Section A – Dead control/heater

If the control is completely dead, there are 2 possible issues:

  • There is no power to the heater or controller.
    1. Disconnect the main loom plug from the heater. This is outside the heater on some heaters and inside the top cover of the heater on others. Once disconnected, use a multimeter to test the voltage on the main supply wires by putting the multimeter probe into the main loom socket where the thick black and red or black and brown wires are located. As an alternative, use the sharp tips of the multimeter probes to pierce the power wires to test for power. If there is no power, start looking for the break.
    2. Check the fuse is ok. Pull it out and test or replace. The fuse will be located in the main cables near your battery or beside the heater. Belief heaters have 1 20A fuse. Eberspacher have 2 fuses, a 5A fuse for the controller (on the thin wires) and a 20A fuse for the heater (on the thick wires).
    3. Check all plugs and connections. Rough roads or movement of other things you are carrying can cause connections to come loose. Trace the wires from the heater to the battery. Check all the connectors are closed or tight. Wobble things and see if the power comes back on.
    4. Check for dislodged pins inside the plugs. Sometimes the little metal pins in the plugs can push back, making a poor contact. Pull apart the connectors and check each wire going into each connector has a metal pin that is fully pushed forward in the plug. Check this at the main plug, but also at the plug where the controller connects to the loom, this plug will be approx. 30cm back down the loom from the controller.
  • If you have power (12V) at the heater and controller and the whole thing is still ‘dead’ there is an issue with the controller or heater ECU. Call Dieselheat to discuss options to service/repair the heater.

Section B – Heater starting but not igniting, no error code before attempting to start.

Heater turns on, blows cold air, the fuel pump ‘ticks’ and tries to start over a period of about 6 minutes, but does not ignite or blow hot air. The heater then shows an error, for example E10 (Belief digital controller); red LED flashes continuously with no pauses (Belief rotary controller); Error 52 or 53 (Eberspacher D2); ‘Fuel Supply or Pump’ (Eberspacher D2L).

The most likely causes of heaters attempting to start but not igniting is no fuel, insufficient power or no combustion air flow – or the heater needs a service.

For heaters which have been running reliably and have suddenly stopped, we suggest check Fuel first. For heaters that always struggle to start, take more than one attempt to start but do start eventually or struggle to start in cold conditions or just struggle in the morning check Power first. For heaters that have not been used for a while, have been in storage, have been towed through mud/dust or been in mud wasp areas check Air and Exhaust and then Fuel.


  1. Check your batteries are fully charged, ie over 12.8V, connect to power or put a battery charger on them to rule out low battery voltage as the issue.
  2. Make sure the heater is connected directly to the battery with the original wiring loom that was supplied with it or with 6mm2 automotive cable. If the heater is connected via any fuse boxes, existing thinner wiring or power switches, bypass all this and connect directly to the battery, ensuring the supplied fuse is still in place. 
  3. If the wiring loom has been extended, make sure it is extended with 6mm2 wire and the join is properly soldered. This heavy duty wire is needed as heaters need a lot of power to start, especially in cold weather. 

Use a multi-meter to measure the voltage at the heater when the heater is trying to start. When starting the heater will pull around 6-10A, it is really important to measure when the heater is starting as glow plug load will cause voltage drop in the wires or battery and if the voltage is below around 12.5V at the heater this will make it hard for the heater to start. Access to measure the voltage can be tricky – sometimes you can insert the multi- meter probes into the back of the heater main connector plug or in some cases it is necessary to pierce the power wires with the sharp tips of the multimeter probes right next to the heater.

If the battery voltage is ok (say over 12.8v) but the voltage at the heater is low (say under 12.4V) so voltage drop between the batteries and the heater is more than 0.4V the cabling may not be correct. Fix and try to re-start the heater. If the heater has a good power supply voltage over 12.5V at the heater when starting and still will not start go to Fuel below.


If the heater is new or ran out of fuel it can take up to 6-8 complete start cycles to purge the fuel line of air and get fuel into the heater. A start cycle takes about 6 minutes and consists of 2 attempts to start by the heater, after which time it will show a fault code. 

A good indication of whether a heater is getting fuel can be white smoke from the exhaust when it is trying to start. If the heater has small amounts of white smoke coming from the exhaust as it tries to start, generally this indicates it is getting fuel, alternatively if there is no smoke generally this indicates there is no fuel.

  1. If the heater has tried to start twice and is showing a failure to start fault code on Belief heaters remove the fuse and put it back to reset, on Eberspacher turn the heater off and on again via the controller. Turn the heater back on and allow it to repeat it’s start cycle. Repeat and try this up to 8 times.
  2. If you have tried to start the heater 8 times and it hasn’t started, disconnect the fuel line from the bottom of the heater and then get someone to turn it on. When you hear the fuel pump tick, about ¼ of a teaspoon of diesel should come out of the fuel line for each tick. If there is no fuel, go to Fuel Fault Finding below. If there is fuel being delivered to the heater, read the Water in Fuel section below. Note the newer Eberspacher fuel pumps are very quiet so it may not be possible to hear them tick, you may need to feel the pump to determine when it is pumping.

Water in Fuel

Water can get into fuel tanks from general atmospheric condensation, leaking fuel caps or from the fuel source. Signs of water in fuel can be the heater spitting or popping when it runs or just not starting. If the fuel pump is pumping check that diesel and not water is coming out of the fuel line. This can be done by draining some fuel into a glass jar, or allowing the pump to pump fuel into a glass jar. Diesel floats on water so you are checking for cloudy fuel or small bubbles of water at the bottom of the jar. If the fuel has water in it, replace the fuel and then follow the out of fuel procedure to purge the contaminated fuel from the fuel lines.

If you fuel pump is pumping and the fuel is not contaminated go to the Air and Exhaust or Power sections.

Fuel Fault Finding

Before proceeding check that when the heater is starting that the fuel pump is ticking or for quiet pumps at least it can be felt to be pulsing. The fuel pumps work on an electromagnetic piston. If the pump gets dirt in it or contaminated fuel the piston can jam so the pump will not tick or pulse. In this situation in most cases the pump will need to be replaced.

If the pump is ticking or pulsing and you have removed the fuel line from the heater and no fuel comes out when the pump ticks, then you have no fuel flow to the heater.  Check the following, and at the end of each test re-check the fuel flow by turning on the heater with the fuel line disconnected and seeing if about ¼ of a teaspoon comes out of the fuel line each time the pump ticks.

  1. If this is a new install, check that your fuel tank is not more than 2m below the fuel pump. This is the allowable limit for the fuel pump to suck up the fuel.
  2. Double check your fuel tank has fuel in it. If you are in a motorhome with fuel coming from the main vehicle tank, check it is more than ¼ full, as the fuel pickups often do not go to the bottom of the tank.
  3. Check your fuel is not old. If you have a white plastic tank, sunlight will allow algae to grow in the fuel and this can block the fuel lines or pump. If this has happened, drain your tank and lines and replace the fuel and fuel tank.
  4. Check all the fuel connections between the tank and the fuel pump. Even the slightest air leak on the inlet side of the pump will allow air into the fuel line and stop fuel flow. Re-test for fuel flow after you check all the connections.
  5. Eberspacher and newer Belief fuel pumps have a tiny strainer built into the inlet side of the pump. This is behind a 17mm nut on the pump inlet. Remove the 17mm nut and pry out the strainer – check it is not blocked.
  6. If you didn’t find a fault with steps 1-5 above, place a short length of fuel line on the inlet of the pump and put it in a small, close temporary fuel supply – like a clean tin of fuel. Turn on the heater and see if the pump is sucking fuel up the fuel line and if it is pumping. If it is pumping re-check points 1-4 above as there must be an air leak or blockage upstream of the pump. If it is not pumping it is likely the pump is faulty or blocked and will need to be replaced.

If you have been able to re-establish fuel flow to the heater, reconnect the fuel line to the heater and re-start the heater.

Air and Exhaust 

If the heater has fuel getting in and a good source of power but is still not starting, you should remove the air inlet pipe and exhaust pipe from the bottom of the heater to check they are not blocked.

For heaters that have been stored or not sued for a while mud wasps love the inlet and exhaust pipes and they can build mud nests right up inside the heater.

After the air inlet pipe and exhaust pipe are removed, try and start the heater a couple of times. If the heater starts after these pipes are removed you should wash out the inlet pipe with water and check the condition of your exhaust. If it is black and full or carbon, you should replace it. If it is just full of mud, wash it out and put it back on.

Still not going?

If your heater has plenty of power, fuel and air and it still won’t start the likely scenario is that is is sooted up inside, so it’s time to call Dieselheat and talk to us about posting it back for a service. See the section on servicing here for instructions.

Section C – Error code before starting

If a heater is not starting and is showing an error code almost immediately when you turn it on, retrieve the exact error code and check the error code listings at the bottom of this guide. 

For Belief heaters an error code on the digital controller will be ‘E” and a number. On the rotary controller, the red LED will flash between 1 and 13 times with a pause between groups of flashes.

For Eberspacher D2 heaters you can load the error codes into the heater by holding down the power button until the controller light flashes, then the error codes will display on the screen.

For newer Eberspacher D2L heaters error codes can be retried by placing the heater in workshop mode. See here for the instructions on how to retrieve error codes.

Heaters with error codes which come up immediately (before they try and start) generally have some sort of failed internal component, for Eberspacher and Belief heaters please call Dieselheat to discuss repairing your heater.

Section D – Error code after starting

If the heater was operating, then stopped and shows an error code, check the list of codes at the end of this guide. In many cases, the error code will relate to over temperature. If this is the case, check section E.

Section E – Heater will not speed up or shuts down and restarts during operation.

Firstly, heaters operating on a thermostat will shut down and restart to maintain the correct room temperature – this is normal operation. Heaters in a room which is above their set target temperature will also either not start or shut down.

If your heater runs but will not speed up, or shuts down after running for a while, but then restarts, but the room is not yet at the heater’s target temperature chances are it has overheated. This can be caused by two issues, blocked airflow or excessive inlet air temperature.

Blocked Airflow: Older heaters can gather dust, dog hair etc. on there main ventilating fans, this can slow down the air flow through the heater and cause it to overheat. Other causes include excessive ducting length, ducting falling off the heater so it recycles its own hot air, lack of inlet ventilation to the space the heater is installed in limiting airflow or closed off hot air outlet vents.

Excessive Inlet Temperature: If heater inlet air temperature is over approx. 35deg in most cases heaters will slow down or shut down. In some cases you will get an overheating error code. Check that you don’t have a lot of uninsulated hot air ducting in the same cavity as the heater. Even if the room air temperature is cool the actual air going into the heater can exceed 35deg if it is being preheated by the hot ducting. Test this by opening up the cavity the heater is installed in and seeing if it operates properly. If this solves the issue, you will need to insulate the ducting, better ventilate the cavity or add inlet air ducting on the heater.

Section F – Smoking heater

If your heater had trouble starting and then starts, it may smoke for a while whilst excess unburnt fuel which has dripped into the exhaust system is burnt off, this can take 15-20 minutes and is not a concern.

If your heater is smoking all the time, check the air inlet and exhaust to ensure they are not partially blocked. Follow the section called Air and Exhaust above to properly check this.

If you ran your heater on low power settings for excessive periods of time without turning up it can be partially sooted up and can get smelly or smokey. Run the heater on full power for a while, if the heater still smokes after operating for 15-20 mins on full power, it most likely is sooted up inside or needs a service. See information on Diesel heater servicing here.

Sending a Heater to Dieselheat

If you have followed the above steps and your heater still won’t start, or is running but smelly or smokey it is probably going to need to be sent to Dieselheat for a service. For information on diesel heater servicing see here.

Error Codes

Belief Manual Controller Error Codes

Belief Digital Controller Error Codes

Eberspacher Airtronic D2 Error Codes


Warranty Icon

Two Year Warranty

All our products are backed by a two year warranty.

Phone Icon

We're here to help

0418 130 971

Shipping Icon

Express Delivery

Freight calculated on check out.