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At a glance
Symptom triage summary
The windscreen mists repeatedly and resists clearing even with the demister running. Persistent fogging means there is more moisture entering the cabin than the ventilation can remove.
UrgencyUrgency not yet verifiedSeverityHighEvidenceSource notes available
What it may indicate
The windscreen mists repeatedly and resists clearing even with the demister running. Persistent fogging means there is more moisture entering the cabin than the ventilation can remove.
Possible system
HVAC
Urgency
Urgency not yet verified
Can I drive?
Drive with caution
Evidence/confidence
Source notes available
See source & evidence notes below
Short answer
The windscreen mists repeatedly and resists clearing even with the demister running. Persistent fogging means there is more moisture entering the cabin than the ventilation can remove.
SeverityHighDrive?Drive with caution
Can I still drive with this?
Drive with caution
Limit trips; avoid heavy loads and long highway runs.
SeverityHigh
High
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Troubleshooting at a glance
UrgencyHigh
Check firstCheck for a sweet smell and an oily film on the inside of the glass, which points at coolant rather than plain condensation
Can you drive?Drive with caution
Stop driving ifIf it is fogged before you set off, do not move until…
The windscreen fogs up and will not clear
The windscreen mists repeatedly and resists clearing even with the demister running. Persistent fogging means there is more moisture entering the cabin than the ventilation can remove.
What can cause it
Water in the cabin from a leaking seal or blocked drain raising humidity
A leaking heater matrix putting coolant vapour into the cabin air
Air conditioning not operating, since it is the dehumidifier in most demist systems
A blocked or saturated cabin air filter
What to check first
Check for a sweet smell and an oily film on the inside of the glass, which points at coolant rather than plain condensation
Confirm the air conditioning compressor engages when the demister is selected
Check the carpets for damp
Replace the cabin air filter if it has not been changed recently
What this symptom does not tell you
An observation narrows the search; it does not identify which of the possible causes applies to your vehicle. Confirm the cause before any part is replaced — several of the causes above produce an identical symptom.
Possible causes
Water in the cabin from a leaking seal or blocked drain raising humidity
A leaking heater matrix putting coolant vapour into the cabin air
Air conditioning not operating, since it is the dehumidifier in most demist systems
A blocked or saturated cabin air filter
What to check first
Check for a sweet smell and an oily film on the inside of the glass, which points at coolant rather than plain condensation
Confirm the air conditioning compressor engages when the demister is selected
Check the carpets for damp
Replace the cabin air filter if it has not been changed recently
When to stop driving / seek help
If it is fogged before you set off, do not move until the windscreen is completely clear: run the demister with the air conditioning on and wipe the inside of the glass. If it fogs while you are already driving, ease off smoothly, avoid sudden braking or steering, and pull over at the first safe place. Only resume driving once you can see clearly through the whole windscreen; if it clouds again straight away, have the cause found before you drive.