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Climate control, bodywork and interiorUrgency: Garage soon

Car heater not heating: causes and the faults that trigger it

A car heater works with the engine coolant, which passes through a small radiator in the dashboard while an electric fan pushes the air. If it does not heat, the most commonly cited causes are a thermostat stuck open, low coolant because of a leak, a water pump that does not move it properly, air or dirt in the circuit and, if no air comes out, a fan or a fuse. It is worth checking soon because a lack of coolant can end in overheating.[1][2][3][4]

One documented fault causes it in the 2.5 dCi YD25:

What it means

The heater uses heat from the engine coolant, so if cold or lukewarm air comes out, the fault is usually in that circuit (level, thermostat, pump, air or dirt) or in the distribution of air to the cabin. It is worth checking because it can be the first sign of coolant loss.

General explanation from MotorFiable. How it shows up in each engine, with sources, is in the list of faults.

What to do now

Garage soon

Look at the temperature gauge and the coolant level with the engine cold; if the engine is slow to warm up or the level is low, have the thermostat and the circuit checked at a garage without waiting.

12 sourcesHow we work

Most common causes

From most to least likely according to the sources. Under each cause, the part and the related OBD codes.

  1. 1. Thermostat stuck open

    Common cause

    According to RACE, if the thermostat stays open the heater will not work properly, because it depends on the engine temperature, and the car also uses more fuel because it thinks the engine is still cold. Replacing it costs around 200 euros including labour (in Spain).[2]

    Faults with with the thermostat and cooling system

  2. 2. Low coolant due to a leak

    Common cause

    Autopista cites a heater that does not heat as before, or that takes longer to blow hot air, among the symptoms of a radiator with problems: the coolant does not circulate properly, which is usually due to a leak, and a lack of fluid can lead to overheating. RACE places leaks in the hoses, radiator or water pump.[3][5]

    Faults with with the thermostat and cooling system

  3. 3. Water pump that does not move the coolant properly

    Possible cause

    Autopista includes erratic heating among the symptoms of a faulty water pump, due to poor coolant circulation, along with leaks, a sweet smell, metallic noise and the temperature warning light. RACE points out that the pump keeps the engine at a constant temperature and that if it fails there may be overheating.[4][5]

    Faults with with the water pump

  4. 4. Air in the cooling circuit

    Possible cause

    HELLA explains that air pockets appear when the radiator or expansion tank sits level with or below the engine, and that air reduces coolant circulation. As the heater gets its heat from that same coolant (RACE), a poorly bled circuit can also be noticed in the cabin, although HELLA does not mention this explicitly.[1][6]

  5. 5. Blocked or leaking heater matrix

    Possible cause

    HELLA points out that limescale deposits and sealing agents can block the heater matrix, and RACE advises switching the heater on occasionally to avoid blockages. Wikipedia adds that it clogs if the coolant is not changed regularly and that its leaks give themselves away through a sweet smell, misted windows or a wet carpet.[6][7][8]

    Faults with with the heating

  6. 6. Fan, fuse or filter (little or no air coming out)

    Possible cause

    RACE explains, in connection with the air conditioning, that a fuse can leave the circuit dead, that if the fans break no air reaches the cabin and that an unchanged pollen filter accumulates dirt. The heater moves air with a similar electric fan.[1][9]

    Faults with with the heating

  7. 7. Seized air mix motor or flap

    Uncommon

    RACE describes a flap moved by an electric motor that mixes cold and hot air. Wikipedia states that the mixing flaps or their control can seize, for example through thermal expansion.[8][9]

    Faults with with the heating

Depending on what you notice

The heater is weak or slow to warm up

Autopista lists it as a symptom of coolant not circulating properly through the system, something usually due to a leak. RACE adds that, with the thermostat open, the heater performs poorly and the car uses more fuel.[2][3]

Cold air comes out when the heater is switched on

Start with the temperature gauge: if the engine does not reach its working temperature either, RACE points to an open thermostat. If the engine does warm up and the air stays cold, the sources focus on the path of the heat into the cabin: air in the circuit (HELLA), a blocked heater matrix (HELLA, Wikipedia), the water pump (Autopista) or the mixing flap (RACE, Wikipedia).[2][4][6][8][9]

The heater does not work and no air comes out of the vents

If no air comes out, RACE cites fuses and faulty fans as the cause of air not reaching the cabin, although it explains this in connection with the air conditioning. The heater pushes hot air with the same type of electric fan.[1][9]

The heater does not heat and the engine overheats

This is the most serious combination. With a thermostat stuck closed the engine temperature keeps rising and RACE says to stop the car as soon as possible; a lack of coolant due to a leak or a faulty pump can also end in overheating (Autopista), and with the red warning light you must pull over immediately (RACE).[2][3][4][5]

Sweet smell or misted windows when the heater is on

Wikipedia states that a leak in the heater matrix gives itself away through a sweet smell, that of the coolant, or through misted windows, and that it can wet the carpet. Autopista also associates the sweet smell with coolant evaporating on hot parts because of a leak, for example at the water pump.[4][8]

What to check first

  1. Look at the temperature gauge: if the engine takes a long time to warm up or never does, suspect the thermostat; if the needle climbs too high or the red warning light comes on, stop.[2][5]
  2. With the engine cold, check the coolant level in the expansion tank: it should be between the two marks. If you need to top up every few days, there is a leak.[5][10]
  3. Check whether air comes out of the vents. If none does, check the fuses and fan before looking in the coolant circuit.[9]
  4. Watch for a sweet smell, misted windows and a wet carpet: they are signs of a leak in the heater matrix.[8]
  5. Ask the garage to check the thermostat and water pump and, if there is air in the circuit, to bleed it; HELLA describes a vacuum filling device for removing the bubbles.[2][4][6]
  6. Switch the heater on from time to time and change the coolant when due to avoid blockages in the system.[7][8]

Can I keep driving?

Urgency: Garage soon

It depends on what is behind it. If it only heats weakly and the temperature gauge stays normal, none of the sources consulted says you have to stop, but it is worth going to a garage soon: an open thermostat increases consumption (RACE) and a lack of coolant, which may be the cause, leads to overheating and serious failures (Autopista). If the warning light turns red or the temperature rises, stop at once: RACE says to stop as soon as possible with a closed thermostat and to pull over with the red warning light.[2][3][5]

How much it costs to fix

Indicative prices published by the sources cited; they depend on the car and the garage.

RepairPrice
Thermostat, part only (according to RACE, price in Spain)[7]€40
Replacing the thermostat including labour (according to RACE, «around 200 euros», price in Spain)[2]€200
Water pump (according to RACE; labour is extra, price in Spain)[7]€300 – €700

Documented faults that cause it

Specific engine and gearbox faults, from most to least important (severity × frequency). Below each title, how the symptom shows up in that fault.

  1. Source for the symptom in the 2.5 dCi YD25[11][12]

Which cars it appears in

This fault affects the 2.5 dCi YD25. We have documented 2 models from 1 make with some of the affected versions: open yours to see all its faults.

Engines

Cars with the affected versions

Heater not heating: causes in your car

2 cars have several documented faults that cause this symptom. Choose yours to see only those that affect it, by engine and by year.

Nissan 2

Frequently asked questions

Why is my car heater not heating?

The most commonly cited causes are a thermostat stuck open, low coolant because of a leak, a water pump that does not move it properly, air or dirt in the circuit, a blocked heater matrix or, if no air comes out, a faulty fan or fuse. RACE and Autopista cover them in different articles.[2][3][4]

Can I drive with a broken heater?

If the engine stays at normal temperature, the sources consulted do not ask you to stop, but they do say to have it checked soon, because the cause may be a coolant loss. If the temperature gauge rises or the red warning light appears, you must stop: RACE says to stop as soon as possible with a closed thermostat.[2][3][5]

How much does it cost to fix a car heater?

It depends on the part. RACE puts the thermostat at around 40 euros for the part and about 200 with labour, and the water pump at 300 to 700 euros plus labour (prices in Spain). Wikipedia warns that replacing the heater matrix requires taking apart a good part of the dashboard and is expensive, but we have not found a euro price for Spain.[2][7][8]

What if the heater only works some of the time?

Autopista lists erratic heating among the symptoms of the water pump, due to poor coolant circulation, and HELLA explains that air trapped in the circuit reduces fluid circulation. In both cases it is worth having a garage check it.[4][6]

How does a car heater work?

The engine heats the coolant; the hot water passes through a small radiator in the dashboard and an electric fan moves the hot air into the cabin, according to RACE. In modern cars, an electronic system regulates the mix of hot and cold air.[1]

Other symptoms: climate control, bodywork and interior

All symptoms

Has this happened to you? Tell us which fault it was, at how many kilometres and how much you paid: it helps other owners know what to expect. Report your fault.

Sources

Every fact on this page links to its source. If you find an error, let us know.

  1. ¿Cómo funciona la calefacción de un coche?RACE · Motoring press · published 24 April 2023 · consulted 4 October 2026 · in Spanish
  2. Termostato del coche: síntomas y averíasRACE · Motoring press · published 11 March 2022 · consulted 4 October 2026 · in Spanish
  3. ¿Cómo saber si el radiador del coche está roto? Posibles síntomasAutopista · Motoring press · published 28 April 2025 · consulted 4 October 2026 · in Spanish
  4. Bomba de agua del coche, qué es, función y cuándo cambiarlaAutopista · Motoring press · published 11 June 2026 · consulted 4 October 2026 · in Spanish
  5. ¿Por qué se calienta el coche?RACE · Motoring press · published 24 July 2023 · consulted 4 October 2026 · in Spanish
  6. Lavado y purgado del sistema de refrigeración: InstruccionesHELLA · Manufacturer · consulted 4 October 2026 · in Spanish
  7. ¿Cómo funciona el circuito de refrigeración de un coche?RACE · Motoring press · published 30 May 2023 · consulted 4 October 2026 · in Spanish
  8. Heater coreWikipedia · Other source · consulted 4 October 2026
  9. El aire acondicionado de tu coche no enfría: te explicamos las razonesRACE · Motoring press · published 23 June 2023 · consulted 4 October 2026 · in Spanish
  10. Líquido refrigerante: todo lo que debes sabercoches.net · Other source · published 19 August 2024 · consulted 4 October 2026 · in Spanish
  11. Tips for buying a second-hand pickup: Nissan NavaraFarmers Weekly Interactive · Motoring press · consulted 5 October 2026
  12. YD25 Engine Guide: Specs, Reliability, Problems & Buying TipsAutopartswd · Technical documentation · consulted 5 October 2026