Frost and starting: what to do when the car will not start in winter

Frost does not create new faults — it exposes existing ones. A car that starts at minus twenty started fine in summer too; a car that cranked sluggishly in autumn will not start in winter.

Why winter is harder

Battery capacity falls with cold. In hard frost the available output is noticeably lower than in summer.

Oil thickens — the crankshaft needs more effort, so the starter needs more current exactly when less is available.

Fuel vaporises worse on petrol; on diesel it also thickens.

Three factors multiply, which is why a car that has been cranking sluggishly for months “suddenly” stops starting.

The pause rule

The most important practical rule of winter.

Crank for no more than 5 to 10 seconds per attempt. Then pause at least 30 seconds.

Why: the starter heats up badly and can burn out; under continuous load the battery loses output and has no time to recover chemically.

How many attempts: three or four. If it will not start, look for the cause rather than finishing off the battery. A fifth consecutive attempt will not start the car but will make the next morning worse.

What to do before trying

Switch off every consumer: lights, heated screens, stereo, air conditioning. Each takes current from the starter.

Switch the lights on for 20 to 30 seconds before cranking in hard frost — a small load warms the battery slightly, chemically.

Depress the clutch on a manual — that removes the drag of thick gearbox oil from the crankshaft. It makes a noticeable difference in frost.

On a diesel, wait for the glow plugs. The coil symbol should go out. In hard frost it is worth cycling twice: ignition on, wait, off, on again, wait; covered in the glow plug article.

Jump starting

Connection order: positive to positive, then the donor’s negative to an earth point on the recipient — not to the battery terminal. That reduces the risk of a spark over the battery.

Let the donor run for a few minutes before attempting.

Do not run both engines at once.

Disconnect in reverse order.

A jump pack is more convenient because it needs no second car and does not depend on somebody else’s wiring.

If it cranks but will not fire

The starter turns briskly — the battery is not the problem.

On petrol: plugs, coils, fuel pump; the symptoms are covered in the cold rough running article.

On diesel: glow plugs, waxed fuel, air in the fuel line.

Common to both: the immobiliser not recognising the key — covered in the immobiliser article.

What to watch on a scanner

Cranking voltage — the key parameter. A deep sag means the battery rather than the starting system.

Coolant temperature — the module must know the engine is cold; a lying sensor prepares the wrong mixture.

Glow plug codes on a diesel.

Immobiliser codes — if the starter does not turn at all.

In short

• Frost does not create faults, it exposes them.
• Crank 5 to 10 seconds, pause 30, and no more than three or four attempts.
• Switch consumers off and depress the clutch before cranking.
• The donor’s negative goes to an earth point, not the battery terminal.
• Cranking briskly without firing means the battery is innocent.

Download the app — cranking voltage, temperature, and codes are read with an ELM327 adapter over Bluetooth SPP on Android.