Why your car won’t start when it’s cold outside—and how to fix it

A weak or dead battery is one of the most common reasons a car won’t start during the colder months. If your car’s battery is weak, the engine will turn over slowly when you try to start it. If it doesn’t start right away, it’ll eventually stop turning over altogether as the last of the energy is drained from the battery.

If the battery is completely dead, you might hear a clicking sound when you try to start it, but the engine won’t turn over.

Cold temperatures reduce a battery’s output by slowing down the chemical reaction taking place inside it, so less power is available to start the engine. Plus, engine oil gets thicker when it’s cold, making the engine harder to turn over, requiring more power from the battery.

The combination of these factors can cause a battery that was weak but still working fine to suddenly fail, leaving you stranded with a car that doesn’t start.

To get your car running again—at least temporarily—you’ll likely just need a jump start. For that, you’ll either need a battery booster pack, or a set of jumper cables and a friend with a running car.

Check your owner’s manual for jump starting instructions. Be sure to follow the proper procedure – this will help keep you safe and ensure you don’t damage your vehicle or anyone else’s in the process.

Once your car is started, keep it running for at least 20-30 minutes so the alternator can recharge the battery. If your battery continues to die, it may need to be replaced.

Since cold weather weakens car batteries, it’s a good idea to have yours tested before the weather drops below freezing. Take your car to a nearby auto parts store or a mechanic—many places will test the battery for free! If the test shows your battery is on its last legs, you’ll want to replace it before cold weather arrives.

Keep in mind, car batteries typically only last three or four years. If yours is getting to that age (you can tell by looking at the date stamped on the top), you’ll want to have it tested more frequently to avoid being stranded, or simply replace it before the next cold season.

If testing shows that your car’s battery is fine but it continues to die, it may not be getting enough of a charge from the alternator. In that case, you’ll want to check the alternator next.

The alternator charges the battery anytime the engine is running. In cold weather, the alternator must work harder to keep the battery charged since the battery is weaker when it’s cold.

It may be time to have your alternator checked if:

• Your battery is weak or dies again and again but tests show that it should be in working order

• You jump start your car but it immediately dies after the jumper cables are disconnected

• The battery warning light on your dash comes on when the car is running

The same places that test your battery—mechanics and auto parts stores—should be able to test your alternator as well. If the test shows that it’s bad, you’ll need to have it replaced.

If you have your alternator tested and the results show that it’s bad, you’ll need to get a new one. Alternators are relatively easy to replace, but they’re connected to the car’s electrical system and are typically run by an engine belt, so it’s a good idea to have the job done by a professional mechanic.

If the battery is good but your car still won’t start, the fuel system may be to blame. Over time, water can build up in the fuel tank due to condensation. If this happens, the water can get into the fuel line and freeze, blocking fuel from getting to the engine.

The most surefire way to get your engine running again if the fuel line is frozen is to move the car inside to a heated space and let it warm up. This may be difficult if your car won’t start, though, so there are other options you can try first.

Buy a bottle of gas line antifreeze or fuel system conditioner—found at most auto parts stores, hardware stores, and gas stations for a few dollars per bottle—and add it to your gas tank. Fill the tank the rest of the way with high quality gasoline and try to start the car again.

You can also try heating your car from outside by placing a bucket or pan of hot water under the fuel line to try to thaw it out enough to get your car running. Do not pour hot water directly on to any frozen part of your car.

If these methods don’t work, it’s time to call a tow truck and have your car moved into a heated garage.

The easiest way to keep your car’s fuel system functioning properly in cold weather is to keep your gas tank full as much as you can. The more gas that’s in the tank, the less space there is for condensation to build up when temperatures fluctuate, keeping water content to a minimum.

If you drive a flex fuel vehicle that can run on E85 (85% ethanol, 15% gasoline), it’s a good idea to use regular gas when it’s cold out. While E85 is technically resistant to freezing, it’s harder to ignite in cold weather, so your engine may not start as easily when using it.

When you try to start your car, the engine may turn over slowly and then stop. This could be because the battery is weak, but part of the problem might also be that the engine oil is too thick. If you have your battery and alternator tested and both are fine, cold engine oil could be the culprit.

Cold weather causes oil to thicken, making it harder to pump and causing additional strain on the starter and battery when trying to start the engine.

If you suspect thickened engine oil is to blame for your car not starting in cold weather, the first thing you should do is check your oil and top it off if it’s low. While you’re checking it, see if it seems to be a normal consistency, or if it seems thick or sludgy.

If it seems too thick, or if it's not quite right, take your car in for an oil change. If you’re the do-it-yourself type and want to save some money, you can get started performing maintenance at home.

Whether you take it to a mechanic or change the oil yourself, make sure the right type of oil is used in your car’s engine. Engine oils vary by viscosity, or resistance to flow. The higher the viscosity, the harder it will be for your engine to pump the oil. Each engine is designed to run on certain oil viscosities, so check your owner’s manual to ensure you’re using the right one.

To keep yourself safe on the road during cold weather, try to prevent these issues before they arise so you won’t be stranded. There are steps you can take in the fall to make sure your vehicle is ready when cold weather hits.

Now that you know how to make sure your car starts in the cold, make sure you have the right car insurance coverage, too.