Why Your Car Won’t Start in the Cold (and What to Do)

Nine times out of ten it’s the battery — but the reason the battery is flat is the part worth finding. Cold weather doesn’t so much cause a no-start as expose one that was coming anyway. Here’s how to read what the car is doing, what to try, and when to stop trying.

Why cold mornings are the breaking point

Three things happen at once at –20°C. The chemical reaction inside the battery slows down, so it can deliver a lot less current than it could in July. The engine oil thickens, so the starter has to work harder to turn the engine. And the engine itself needs a richer mixture and more cranking to catch. A battery that was quietly at 60 per cent of its capacity all summer meets the hardest job of its year on the coldest morning, and loses.

What the symptom tells you

• Slow, laboured cranking that fades — almost always the battery. Either it’s worn out, or something drained it overnight, or the alternator wasn’t recharging it properly on your short trips.
• One click, or rapid clicking, and nothing — the battery has enough to work the starter solenoid but not to turn the engine. Battery again, or a badly corroded cable end. Occasionally the starter itself.
• Total silence, no dash lights — dead battery, or a loose or corroded terminal. Check the connections first.
• Cranks strong but won’t catch — this one isn’t the battery. Fuel delivery, spark, or a sensor. On an older vehicle, moisture frozen in a fuel line. Boosting won’t help; see our engine diagnostics page.
• Starts, then stalls — usually fuel or idle control, sometimes a very weak charging system. Also not a boost problem.

Boosting safely

If it’s the battery, a boost gets you going. Red to the dead battery’s positive, red to the good battery’s positive, black to the good battery’s negative, and the last black clamp to bare metal on the dead car’s engine or frame — not the battery’s negative post. Start the good car, wait a few minutes, then try the dead one. A modern vehicle has a lot of electronics riding on that circuit, so take the time to get the order right; a reversed connection is an expensive mistake.

A lithium booster pack in the trunk does the same job without a second car. It’s the single most useful item in a winter emergency kit.

It started after a boost. Is it fixed?

No — you’ve proved the rest of the car works. You haven’t learned why the battery was flat. Drive it for a while so the alternator can recharge it, then have the battery and charging system tested. If the battery holds a charge and the alternator is putting out what it should, it was probably a one-off. If not, you’ll be boosting it again on the next cold morning, usually somewhere less convenient.

Heading it off

The fix is boring: test the battery each fall once it’s three years old, keep the terminals clean, and if you have a block heater, use it. If the vehicle mostly does short trips, an overnight trickle charger every couple of weeks in winter keeps the battery topped up. Our winter battery care guide goes into more detail, and here’s when a block heater is worth using.

Common questions

How cold is too cold for a car battery?

A healthy battery starts a car fine at –30°C. The cold isn’t the problem; it’s the margin. A worn battery that has lost a third of its capacity starts fine at 5°C and fails at –15°C.

Does idling to charge the battery work?

Barely. At idle the alternator produces a fraction of its output. A 30-minute drive at road speed does more than an hour of idling in the driveway.

My battery is new and it still won’t start. Why?

Either the alternator isn’t charging it, something is draining it while the car sits, or the problem was never the battery. A charging-system test sorts out which.

Should I replace the battery before it fails?

If it’s five years old and tests weak in the fall, yes — replacing it on your schedule is cheaper than replacing it on a tow truck’s.

This article is part of our guide to winter driving in Oxford County. Rick’s Automotive tests and replaces batteries, starters and alternators in Woodstock — see our electrical and batteries page or book a test.