Car won't start in cold weather: causes and what to do

Likely causes, most common first

1. A weak or old battery

   By far the most common cause. The RAC calls the battery "by far the most likely" culprit, and the AA says battery problems were the top cause of its winter callouts, at 24%. Cold slows the battery's chemistry, so it holds and delivers less power. AAA's figures: about 35% less strength at 32°F (0°C) and about 60% less at 0°F (-18°C), while a cold engine can need up to twice the normal current to start. Signs: slow, labored cranking, dim lights, or a clicking sound. Batteries over three years old are most at risk.

   Read the fix guide →

2. Thick engine oil

   Oil flows less freely when it is cold, so the engine is harder to turn and the battery and starter work harder. Oil that is too thick for the climate, or old and overdue for a change, makes it worse. Use the grade in your owner's manual; many cars list a thinner winter option.

3. Diesel only: waxed fuel or glow plug trouble

   In very cold weather diesel can turn waxy and block the fuel filter or injectors, according to the AA. Diesels also rely on glow plugs to heat the cylinders for a cold start. Wait until the coil-shaped glow plug light on the dashboard goes out before you crank. If that light flashes or stays on, a glow plug or its control unit may have failed.

4. Water in the fuel freezing

   Moisture can condense in the tank and freeze in the narrow fuel lines, so the engine cranks normally but will not fire. The RAC notes that keeping the tank close to full reduces condensation. An empty or nearly empty tank can also stop a car starting on a cold day.

5. A worn starter motor or alternator

   If you hear a single click and the lights and electrics work normally, the starter may be the problem, and a jump start will not help. If the car starts with a jump but dies again soon after, or the battery keeps going flat, the RAC points to the alternator.

   Read the fix guide →

What to do right now

1. Switch everything off. Lights, heater fan, heated seats, radio and phone chargers all draw from the battery.
2. Press the clutch if your car is a manual, then turn the key or press start. On a diesel, wait for the glow plug light to go out first.
3. Crank for about 10 seconds, no longer. If it does not start, wait 1 to 2 minutes so the battery recovers, then try again. Long cranking can overheat the starter.
4. After two or three failed tries, stop. Jump start the car with cables or a portable jump starter, following our safe jump-start guide. Never try to jump or charge a battery that looks cracked, swollen or frozen: the RAC advises replacing a battery that has frozen.
5. Call your breakdown service if a jump start does not work, if the engine cranks normally but will not fire (likely fuel), or if you are not comfortable doing it yourself.
6. Once it is running, drive for about half an hour if you can, rather than a short hop, so the alternator can recharge the battery. Book a battery test in the next few days.

When to stop driving: if the battery warning light comes on while you are driving, the charging system is not working and the car is running on what is left in the battery. Head somewhere safe and get it checked before you switch off. See battery warning light.

What it costs to fix

Most cold-start problems end with a battery test and, if it fails, a new battery. See our car battery replacement cost guide for current prices. Many parts stores and breakdown services test batteries for free. A starter motor or alternator is a bigger bill; see the starter motor and alternator cost guides.

DIY or garage?

How to prevent it

• Test the battery before winter, especially if it is over three years old. Replace it on your own schedule rather than at the roadside.
• Use the oil grade in your owner's manual, and do not let an oil change run overdue going into winter.
• Keep the fuel tank fuller in winter to cut condensation. Diesel drivers should fill up locally in a cold snap so they get the winter blend.
• Park in a garage if you have one. Even a few degrees warmer helps the battery.
• Drive longer trips now and then. The AA suggests regular runs of half an hour or so; if you only do short trips, the RAC suggests charging the battery about once a week in winter.
• Switch off lights and accessories before you turn the engine off, so they don't load the battery at the next start.
• Use a block heater where it is common. In very cold places such as Canada, the northern US and Scandinavia, many cars and trucks have one. As an example, Ram's owner's manual for its 2500 and 3500 pickups recommends plugging in below 0°F (-18°C), and for at least an hour. Check your owner's manual for your car.
• Get ready for the rest of winter too: see how to defrost a windshield fast and winter tires vs all-season tires (winter tyres, in the UK).