Should You Warm Up Your Car in Winter? Idling and the Law

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

Do modern engines need warming up?

The habit comes from older cars with carburettors and chokes, which ran badly until they were warm. Fuel injected engines adjust the mixture automatically, so they run properly from the first few seconds.

An engine actually warms up much faster when it is working gently than when it is idling. Sitting on the drive for ten minutes uses fuel, puts more unburnt fuel and moisture into the oil, and on a diesel adds soot that the particulate filter has to deal with later. If your car already struggles with short trips, our guide to the DPF warning light explains why.

The better routine is simple:

1. Start the engine and set the heater to the windscreen with the air conditioning on.
2. Clear the glass, mirrors, lights and number plates with a scraper and de-icer while you stand by the car.
3. Once you can see clearly, drive off at moderate revs for the first few miles.

Classic cars with a choke are a different story. They may need a short spell on choke, but you should still stay with the car.

What the law says

Highway Code rule 123 says you must not leave a parked vehicle unattended with the engine running, or leave the engine running unnecessarily while the vehicle is stationary on a public road. The "must not" wording means it is backed by law, in this case the Road Vehicles (Construction and Use) Regulations.

In practice this means:

• Leaving the car running on the street while you go back indoors is not allowed.
• Sitting in a parked car on the road with the engine running for no good reason can also count.
• Councils in England have powers to issue fixed penalties for unnecessary idling, and some actively enforce it, particularly outside schools.

Rule 229 adds that you must be able to see properly before you set off, so clearing only a small porthole in the windscreen is not acceptable either. Our guide to defrosting a frozen windscreen covers quick, safe methods.

The theft and insurance risk

Cars left running with the keys inside are a well-known target on cold mornings. It takes seconds to drive one away, even from a driveway. Many insurance policies exclude theft where the keys were left in the vehicle or it was left unattended and running, so you could lose the car and the claim. Read your policy wording if you are unsure.

Keyless cars bring their own risk. Some will keep running if the key is taken out of range, and will not stop until switched off.

Better ways to get going on frosty mornings

• Cover the windscreen overnight with a proper screen cover or an old blanket held in by the doors.
• Park facing east where you can, so morning sun helps.
• Keep washer fluid at winter strength so it does not freeze on the glass.
• Use a pre-heater or app if your car has one, as these are designed for the job.
• Allow five extra minutes so you are not tempted to set off half-cleared.

For a broader list of jobs before the cold sets in, see our winter car checklist and the local advice on Kent winter roads.

When a slow start is a sign of a fault

If you find yourself idling because the car runs roughly when cold, demists poorly or the heater takes ages to blow warm air, that is worth investigating. Common causes include a thermostat stuck open, a tired battery, a blocked pollen filter or a heater matrix that is partly blocked.

How we can help

We visit homes and workplaces across Aylesham, Canterbury, Dover, Deal and nearby villages. We can check the battery and charging, cooling system, thermostat and cabin filter, and read fault codes if the car runs badly when cold. Labour is £30 to £50 per hour depending on the job, and you get a quote before work starts. Send your reg and postcode on WhatsApp 07563 350335 or use our contact page.

Frequently asked questions

Is it illegal to leave your car running to defrost?

On a public road, Highway Code rule 123 says you must not leave a parked vehicle unattended with the engine running or leave the engine running unnecessarily while stationary. You can be fined, and an unattended running car is an easy target for thieves.

How long should I warm up my car before driving?

For a modern car, only as long as it takes to clear every window and light properly, then drive off gently. The engine warms up faster under light load than at idle.

Does idling damage a modern engine?

Occasional idling will not, but long periods of cold idling let fuel and moisture build up in the oil and, on diesels, can add soot to the DPF. Gentle driving is better for the engine.

Will my insurance pay out if my car is stolen while defrosting?

Many policies exclude theft when the keys are left in or near an unattended car. Check your policy wording, but assume you may not be covered.

Sources

• The Highway Code: rules for drivers and motorcyclists (103 to 158), GOV.UK
• The Highway Code, GOV.UK