Is it illegal to leave your car running to defrost it? The law, fines and insurance

What the law says about a running engine

The rule is Highway Code rule 123, and it is a MUST: “You MUST NOT leave a parked vehicle unattended with the engine running or leave a vehicle engine running unnecessarily while that vehicle is stationary on a public road.” It adds that if you are likely to be stationary for more than a couple of minutes you should apply the parking brake and switch off, and that running the engine is permissible in traffic or for diagnosing faults. Defrosting is not mentioned either way.

Two regulations sit behind it. Regulation 107 of the Road Vehicles (Construction and Use) Regulations 1986 says no one may leave a motor vehicle on a road “which is not attended by a person licensed to drive it unless the engine is stopped” and the parking brake set. Regulation 98 requires the driver of a stationary vehicle to stop the engine, and any other machinery on it, “so far as may be necessary for the prevention of noise or of exhaust emissions”. The Code, which applies in England, Scotland and Wales, marks legal requirements with MUST and says disobeying one is a criminal offence. Northern Ireland’s own Highway Code has the same rule 123, backed by its own regulations, so the fines on this page are for Great Britain.

Walking away while it defrosts

Leaving the car unattended has no minimum time in either rule 123 or regulation 107, and the regulation’s only exceptions cover vehicles being used for fire and rescue, police or ambulance purposes, and vehicles parked safely whose engine is needed to drive machinery forming part of, or mounted on, them. Defrosting is not listed. Starting the car on the street and going back indoors while the windscreen clears is exactly what rule 123 prohibits. The Code’s own security advice is to remove the key and lock the car “even if you only leave it for a few minutes”.

On your own drive the legal position is less clear-cut. Regulation 107 applies “on a road”, and the Road Traffic Act 1988 defines a road in England and Wales as “any highway and any other road to which the public has access”, with a similar definition for Scotland. The Highway Code says that can take in “many roadways and driveways on private land”, but a private drive the public has no access to is generally not a road, and whether yours counts depends on the facts. The theft and insurance points below apply wherever the car is parked.

Sitting with it while the glass clears

Staying with the car changes the question to whether the engine is running “unnecessarily”. Rule 123 does not define the word, and regulation 98 does not mention defrosting. The official guidance we found that does mention it treats running the engine to clear the glass before a journey as a legitimate reason. The Welsh Government lists “de-icing and de-misting windows for safety before making a journey” among the occasions you may need to idle, and Newcastle City Council gives defrosting a windscreen for a few minutes as an example of when drivers will not be penalised. That is guidance, not an exemption written into the law.

Warming up for comfort is treated differently: the Welsh Government lists “operating heating/cooling systems for comfort” as an unacceptable reason. In practice, run the engine while you scrape and brush, then set off once every window is clear, rather than sitting in a warm, parked car. And the person with the car has to be a driver: regulation 107 only counts it as attended by someone “licensed to drive it”.

Idling fines: who issues them and how much

In England, the Road Traffic (Vehicle Emissions) (Fixed Penalty) (England) Regulations 2002 let councils authorise officers to tell the driver of a vehicle stationary on a road to switch off, and to issue a fixed penalty for needless idling. The penalty is £20, rising to £40 if it is not paid in time. Refusing an officer’s request to switch off is a separate offence, with a fine of up to £1,000 if it goes to court.

Enforcement depends on your council. Those that do it, such as Newcastle and Kensington and Chelsea, describe asking drivers to switch off first, with the £20 penalty for those who refuse. Scotland’s regulations also set £20, rising to £40 if unpaid. In Wales the £20 penalty is replaced from 30 September 2026: councils set their own amount, or a scale, between £75 and £150, and must have published it on their website before an offence is committed.

The police can act too, because breaking regulation 98 or 107 is an offence under section 42 of the Road Traffic Act 1988. It can be dealt with by a £50 fixed penalty, with no penalty points, or by a fine of up to £1,000 for a car if it goes to court.

If the car is stolen: the insurance question

Motor policies can exclude theft when the keys were left in or around the car, or when it was left unattended, and the Financial Ombudsman Service lists both among the reasons insurers turn down theft claims. When a complaint reaches it, the Ombudsman checks that the exclusion was clearly highlighted when the policy was sold, and considers how close the driver was to the car and whether they could reasonably have stopped the theft.

In one published case, a driver left his car on the drive with the keys in the ignition and the engine running to de-ice it, and it was stolen while he was upstairs. The Ombudsman found the insurer had acted fairly in refusing the claim: the car was out of sight, and the “keys in car” exclusion had been clearly highlighted. Staffordshire Police say thieves look out for vehicles left running on frosty mornings, and on one morning in January 2026 two vehicles left to defrost were taken from driveways in Stoke-on-Trent. Check your policy for a keys-in-car or unattended-vehicle exclusion before the first frost.

Clearing the car: what rule 229 requires

Rule 229 lists what has to be clear before you set off: “you MUST be able to see, so clear all snow and ice from all your windows” and “you MUST ensure that lights are clean and number plates are clearly visible and legible”. It adds that mirrors should be clear and windows demisted thoroughly. The law behind the first line, regulation 30 of the Construction and Use Regulations, requires all glass to be kept so that it “does not obscure the vision of the driver” while the car is driven on a road.

The number plate law is strict. Under section 43 of the Vehicle Excise and Registration Act 1994, a plate that is “in any way” obscured, or not easily distinguishable, is an offence with a fine of up to £1,000, unless you can show you took all reasonably practicable steps to keep it clear. A letterbox scraped in the windscreen does not meet rule 229, and the RAC’s advice is never to drive with only a small viewing hole cleared. Once the car is clear, our guide to driving in snow and ice covers the Highway Code’s rules for the road itself.

Snow on the roof

Rule 229 also says to “remove all snow that might fall off into the path of other road users”. That line is advice rather than a MUST, but the Highway Code says its advisory rules can be used in evidence in court to establish liability. As the RAC points out, snow left on the roof can fall onto your own windscreen while you drive or fly into the path of another car.

The quick way to defrost, and what not to do

The AA’s routine keeps you beside the car throughout. Make sure the wipers are switched off before you start the engine: if they are frozen to the glass, the wiper motor can be damaged or the rubber torn. Start the engine, send warm air to the windscreen, switch on the rear screen heater and any heated mirrors, and turn on the air conditioning, which dries the air and stops the glass misting. Then use the time: brush off the snow, spray de-icer and scrape, and do not drive until all the glass is clear. If you need to go back indoors, switch off and lock the car.

Never pour hot or boiling water on the glass. The RAC says boiling water will likely crack it, an effect known as thermal shock, and the AA advises against even lukewarm water, which can still crack the glass, especially if it is already chipped, and can refreeze on the wipers. Never run the engine inside a garage to warm the car either: the NHS says turning on a vehicle engine in an enclosed space such as a garage can cause a build-up of carbon monoxide.

Two habits make frosty mornings quicker. A sheet of cardboard across the windscreen overnight, which the AA suggests, stops it icing up, and the Highway Code’s rule 228 puts de-icer and an ice scraper first in the kit it recommends for winter journeys. Our car kit builder sizes a winter car kit for whoever travels with you, and our winter driving kit page compares the options.