Your Diesel Heater Won't Hold a Temperature. It's Probably Too Big.

The complaint is always described the same way. The heater runs, the van gets too hot, the heater shuts off, the van gets cold, the heater fires up again, and somewhere in that loop you stop sleeping. People arrive at this looking for a fault code.

There usually is not one. In most vans this is a correctly functioning heater that was sized for a different vehicle, and no amount of diagnosis will find a broken part because nothing is broken.

Work through the three causes in this order. They are ordered by how common they are, and the cheap fix only helps with the last one.

We have bench-tested no heater. What follows is manufacturer documentation and the pattern of owner reports, labelled as such.

Cause 1: the heater is too big, and this is not fixable

A diesel air heater regulates by varying its burn rate between a minimum and a maximum. The minimum is not zero. If your van’s heat loss at your target temperature is lower than the heater’s minimum output, the heater physically cannot hold that temperature. It can only overshoot, stop, and restart.

That is exactly what a 5kW unit does in an insulated van that needs 2kW. The cycling is not a malfunction. It is the only regulation strategy available to a machine whose quietest setting is still too loud.

Two things follow, and the second one is the expensive part:

• No controller upgrade fixes this. A better thermostat asking for 18°C from a heater whose floor is 24°C worth of output gets the same cycling, just with nicer buttons.
• Idling is where soot comes from. A diesel heater forced to run at its lowest setting carbons up its glow plug and heat exchanger faster than one running at a steady medium. So an oversized heater is not only failing to keep you comfortable, it is aging itself faster than a right-sized one would. This is covered in more detail in our diesel heater buyer’s guide, where sizing gets the space it deserves.

How to tell this is your problem: your heater is 5kW or larger, your van is insulated, and the cycling happens even in genuinely cold weather at a modest setpoint. If it holds temperature fine at -5°C and cycles badly at 5°C, that is the tell. The heater is only well-matched to your van when it is much colder than you usually camp.

The honest fix is a correctly sized 2kW unit. On 2026-08-26 a 2kW budget all-in-one listed at $96.21, which is less than most people expect to hear after being told to replace a heater, and roughly four times the price of the controller that will not fix this. That is an unwelcome answer when you have already spent the money, and we would rather give it than sell you a controller that cannot work.

Cause 2: the thermostat is reading the wrong air

Assume for a moment the size is right. The next question is what the unit thinks the temperature is.

Many heaters take their reading at the controller, or at the heater’s own air inlet. Neither is the air you are sleeping in. A controller mounted high on a wall near the heater’s outlet sits in the warmest air in the van and will shut the unit down while the mattress is still cold. A sensor at the heater’s inlet, under a bed platform, reads a pocket of air that warms and cools on a completely different schedule to the space.

The result reads exactly like a cycling fault, and it is a placement problem.

Worth checking before you buy anything:

• Where is the sensor? If it is in the controller, where is the controller mounted? High and near the outlet is the worst case.
• Can the sensor be extended or relocated? Some units support a remote sensor. This is a real fix and it is cheap.
• Is the air actually circulating? A heater blowing into a space with no return path warms a bubble, satisfies its own sensor, and shuts down. This is the same physics that makes van ventilation matter for condensation.

The comfort measurement that matters is air at sleeping height, which is also, incidentally, the height at which a carbon monoxide detector belongs, and for a related reason: the number that matters is the one where your head is, not the one where the device is convenient to mount.

Cause 3: the controller does not do temperature at all

This is the cause worth checking first if your unit is correctly sized, because it is the one with a fix.

A significant number of budget heaters ship with a controller that sets power levels, not temperature. You choose a level between roughly 1 and 6, or a Hz value, and the heater burns at that rate until you change it. There is no setpoint. There is no thermostat. The unit is not failing to hold a temperature. It was never asked to.

Owners hit this constantly and describe it as a fault, because every other heating appliance they have ever owned had a thermostat. In this category, thermostat mode is a feature, not an assumption.

How to tell: if your controller shows a number you set and the heater simply obeys it regardless of how warm the van gets, you have a power-level controller. If it shows a target and a current temperature as two different numbers, you have a thermostat.

The fix is a thermostat-capable controller, or a unit that has one. A universal LCD thermostat controller listed at $23.59 on 2026-08-26 and covering 2, 5 and 8kW heaters is the cheapest real fix on this page, and it is the only one of the three causes where twenty-odd dollars genuinely solves the problem. Check it against your heater’s plug and control protocol before ordering, because “universal” in this category means “fits most Chinese units”, not “fits everything”.

When you are choosing a replacement unit instead, look for automatic adjustment, or an app or remote controller that exposes a target temperature rather than a burn level. That combination, on a correctly sized 2kW unit, is what actually holds a setpoint overnight.

If you are replacing the unit rather than the controller, our comparison of the budget and premium tiers covers what the price gap does and does not buy. Short version: it buys documentation and diagnosis, not better temperature regulation.

The diagnosis, in order

1. How many kW is your heater, and is your van insulated? 5kW in an insulated van, cycling in mild weather: you are in cause 1, and the fix is a smaller heater.
2. Where does it read temperature? Controller mounted high, or sensor buried under a platform: relocate or extend before spending anything.
3. Does your controller have a target temperature at all? If it only sets power levels, that is your answer, and a thermostat-capable controller or unit fixes it.

Two of these three are cheap. The first one is not, and it is the most common. That ordering is uncomfortable but it is the honest one.

What we would measure

• Minimum sustained output for 2kW and 5kW units, which is the number that decides whether cycling is inevitable in a given van and which no manufacturer publishes.
• Setpoint accuracy at sleeping height versus at the controller, over a full night.
• Soot accumulation after a season of low-output running against a season of steady medium running, which is the claim everyone repeats and nobody has photographed.

Until we have run those, this page is documentation and owner reports rather than measurement, and we would rather label it that way.