Symptom
Windscreen will not demist
The screen stays misted, or clears very slowly, on the demist setting. Worth knowing why the air conditioning is involved: a heater warms the air but does not dry it, and it is the compressor that condenses moisture out before it reaches the glass — which is why it runs on the screen setting even in freezing weather. So a demisting complaint often points at the climate system or its controls rather than at the heater, and it is a visibility problem rather than a comfort one.
Also described as: wont demist, screen misting up, windscreen fogging, cant clear the screen, demister not working, windows steaming up, windshield will not demist, windshield fogging, windscreen will not defog, wont defog, شیشه بخار میگیرد, بخار شیشه پاک نمیشود, no desempaña parabrisas, luna empañada, no quita vaho, الزجاج لا ينشف, الشبورة لا تزول, الزجاج مضبب باستمرار
What this usually means
Worth knowing why the air conditioning is involved: a heater warms the air but does not dry it, and it is the compressor that condenses moisture out before it reaches the glass — which is why it runs on the screen setting even in freezing weather. So a demisting complaint often points at the climate system or its controls rather than at the heater, and it is a visibility problem rather than a comfort one.
That makes the first check whether the compressor is actually engaging on the screen setting. A system that has leaked down, or one refusing on a false ambient reading, leaves the car blowing warm damp air at the glass — which mists it further before it clears.
The second question is whether the air reaching the screen is outside air. Recirculation traps the moisture the occupants are producing, and a recirculation flap stuck closed makes demisting nearly impossible in wet weather. Most cars deselect recirculation automatically on the screen setting, so one that does not is reporting a flap or its control.
Then airflow. A blocked cabin filter starves the screen vents specifically, because they are at the end of the ductwork — so a filter years overdue shows up as poor demisting long before anyone notices reduced cooling.
And rule out the moisture source, because it is often the real fault. A blocked scuttle or sunroof drain, a failed door membrane, or a leaking heater matrix puts water into the cabin faster than any system can remove it. A film on the inside of the glass that smears rather than wipes clear is coolant, and that is a matrix leak rather than a demisting fault.
Working through it
| What you find | Likely cause | How to confirm it | What fixes it |
|---|---|---|---|
| Compressor does not engage on the screen setting | The system cannot dry the air — low refrigerant, a false ambient reading, or a control fault | Listen for the clutch and watch the idle lift on the screen setting. Read live refrigerant pressure. | Repair the air conditioning fault. This is a visibility issue, so treat it with the urgency that implies. |
| Recirculation stays selected on the screen setting | Moisture from the occupants is being recirculated rather than expelled | Check whether the car deselects recirculation automatically on demist. A flap that does not move is the fault. | Free or renew the recirculation flap actuator. Deselect it manually meanwhile. |
| Weak airflow from the screen vents specifically | A blocked cabin filter — the screen vents are at the end of the ductwork and starve first | Inspect the filter, which is frequently years overdue. Compare airflow at the screen against the face vents. | Renew the filter. Cheapest cause here and the one that shows on the screen before anywhere else. |
| Carpets damp, windows misting overnight | Water entering the cabin — blocked scuttle or sunroof drains, or a failed door membrane | Feel the carpets, especially the footwells and under the seats. A misting problem with damp carpet is a leak, not a climate fault. | Clear the drains and reseal the membrane, then dry the cabin thoroughly. |
| Oily film that smears rather than wiping clear, sweet smell | A leaking heater matrix putting coolant into the airflow | Check the passenger footwell for damp and smell it. The film is distinctive and does not clean off easily. | Renew the matrix. See the coolant-loss entry — this also explains a level that keeps falling. |
| Screen clears slowly but the engine also warms slowly | A thermostat stuck open — there is not enough heat to work with | Watch live coolant temperature. A cold engine cannot supply a hot screen. | Renew the thermostat. Demisting, heater performance and consumption recover together. |
-
Compressor does not engage on the screen setting
- Likely cause
- The system cannot dry the air — low refrigerant, a false ambient reading, or a control fault
- How to confirm it
- Listen for the clutch and watch the idle lift on the screen setting. Read live refrigerant pressure.
- What fixes it
- Repair the air conditioning fault. This is a visibility issue, so treat it with the urgency that implies.
-
Recirculation stays selected on the screen setting
- Likely cause
- Moisture from the occupants is being recirculated rather than expelled
- How to confirm it
- Check whether the car deselects recirculation automatically on demist. A flap that does not move is the fault.
- What fixes it
- Free or renew the recirculation flap actuator. Deselect it manually meanwhile.
-
Weak airflow from the screen vents specifically
- Likely cause
- A blocked cabin filter — the screen vents are at the end of the ductwork and starve first
- How to confirm it
- Inspect the filter, which is frequently years overdue. Compare airflow at the screen against the face vents.
- What fixes it
- Renew the filter. Cheapest cause here and the one that shows on the screen before anywhere else.
-
Carpets damp, windows misting overnight
- Likely cause
- Water entering the cabin — blocked scuttle or sunroof drains, or a failed door membrane
- How to confirm it
- Feel the carpets, especially the footwells and under the seats. A misting problem with damp carpet is a leak, not a climate fault.
- What fixes it
- Clear the drains and reseal the membrane, then dry the cabin thoroughly.
-
Oily film that smears rather than wiping clear, sweet smell
- Likely cause
- A leaking heater matrix putting coolant into the airflow
- How to confirm it
- Check the passenger footwell for damp and smell it. The film is distinctive and does not clean off easily.
- What fixes it
- Renew the matrix. See the coolant-loss entry — this also explains a level that keeps falling.
-
Screen clears slowly but the engine also warms slowly
- Likely cause
- A thermostat stuck open — there is not enough heat to work with
- How to confirm it
- Watch live coolant temperature. A cold engine cannot supply a hot screen.
- What fixes it
- Renew the thermostat. Demisting, heater performance and consumption recover together.
Codes that produce this symptom (8)
Listed with the codes that most typically present this way first — the likeliest place to start.
- U0164 Lost Communication With HVAC Control Module Typical of this code Moderate
- U0324 Software Incompatibility With HVAC Control Module Typical of this code Moderate
- U0231 Lost Communication With Rain Sensing Module Low
- U0245 Lost Communication With Entertainment Control Module — Front Low
- U0256 Lost Communication With Front Controls Interface Module A Low
- U0257 Lost Communication With Front Controls/Display Interface Module Low
- U0424 Invalid Data Received From HVAC Control Module Moderate
- U0532 Invalid Data Received From Rain Sensing Module Low
Information, not instruction
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