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uk Transport & mobility emergencies heater not working in car • defroster not working • demister not working • windows keep fogging up • windscreen fogging while parked • car windows misting up • can’t clear windscreen • fogged windscreen can’t see • condensation inside car • car heater blows cold • car ventilation not clearing mist • stuck parked with foggy windows • safe to drive with foggy windows • visibility blocked in car • damp car interior fog • car climate control failure • front screen demist failed • rear screen demist failed • windscreen keeps misting

What to do if…
your heater or defroster is not working and the windows keep fogging while you are parked

Short answer

Do not drive until you can see clearly through the windscreen and front side windows. Switch to “dry air” (fresh air in, recirculation off, fan high; A/C on if available) and clear the inside glass; if you can’t keep it clear, arrange help instead of setting off.

Do not do these things

  • Do not “just go slowly” with a misted windscreen or side windows — if you can’t see clearly, don’t move off.
  • Do not run the engine in an enclosed space (like a garage), even with the door open.
  • Do not sit for a long time with the car sealed up and the engine running; keep fresh air coming in.
  • Do not use hot water on the windscreen (it can crack the glass) or wipe with anything gritty (it can smear/scratch and worsen glare).
  • Do not keep recirculation on if fogging is getting worse — it often traps moisture inside.
  • Do not open the bonnet and remove caps from a hot cooling system (if the engine is hot/steaming), or reach near moving parts.

What to do now

  1. Make the situation safe to pause. Stay parked. If you’re in a risky place (poor visibility for other drivers, narrow verge, awkward lay-by), only reposition if you have a clear enough view to move safely. If you can’t see well enough to move, stay put and use hazard lights if appropriate.
  2. If you’re in a garage or other enclosed space, get outside first. Turn the engine off, open the door, and move the car outside only once you can see enough to do so safely (or ask for assistance).
  3. Switch to “dry air” settings.
    • Select windscreen demist/defrost mode (if available)
    • Set fresh air intake (recirculation off)
    • Set fan high
  4. Turn A/C on if the car has it (even if heat isn’t working). A/C often helps remove humidity from cabin air. If the heater won’t warm, A/C + fan high + fresh air can still help dry the air and clear fog.
  5. Dump humid air quickly. Crack two windows slightly (a few millimetres to a centimetre) for 1–3 minutes, then reassess.
  6. Clear the inside glass properly (so it doesn’t instantly re-fog).
    • Wipe the inside of the windscreen and front side windows with a clean, dry microfibre cloth (or paper towel).
    • If you use wipes/cleaner, finish with a dry wipe to reduce smearing.
  7. Remove the moisture source. Move wet coats/umbrellas off the dashboard/footwells; bag wet items; if mats are soaked, shake or stand them up to drain when safe.
  8. Re-check whether it’s safe to drive. Only set off if you can keep:
    • the full windscreen clear, and
    • both front side windows clear enough for mirrors and junction checks. If fog returns quickly or visibility stays patchy, do not drive.
  9. If you can’t keep it clear: get assistance.
    • Call your breakdown provider (or a local recovery service) and say: “Heater/defroster isn’t working; windows fogging; visibility unsafe.”
    • If you’re stranded in a dangerous location and feel at risk, call 999.

What can wait

  • Diagnosing the cause (coolant level/leak, heater matrix, blower motor, cabin filter, A/C faults).
  • Any DIY repairs beyond basic cabin clearing.
  • Booking a garage appointment — first priority is safe visibility and a safe next step.

Important reassurance

This problem is common and it can feel urgent because visibility drops fast. Choosing not to drive until you can see clearly is the safest decision, and using “dry air” tactics usually improves things quickly.

Scope note

This is first-steps guidance for the moment you realise the windows won’t stay clear. Once you’re safe, a mechanic or breakdown service can diagnose the underlying fault.

Important note

This is general information, not mechanical or legal advice. Do not drive if you cannot maintain clear visibility. If you notice overheating, steam, a burning smell, or warning lights, treat it as a breakdown and seek professional help.

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