Why Your Car Won’t Start in Cold Weather

Cold attacks from both ends at the same time, and that is the whole explanation.

The battery delivers less, because the chemical reaction inside it slows as temperature falls. And the engine needs more, because cold oil is thicker and the starter has to turn it against that resistance.

Both curves move the wrong way together. A battery that was comfortably adequate at 20°C can be marginal at 0°C and inadequate at −5°C — with nothing having changed overnight except the weather. That is why a car that started faithfully all autumn refuses on the first hard frost. The fault was already there; the cold just stopped hiding it.

On top of that squeeze sit a handful of causes that are specific to cold, and most of them are diesel.

How much the cold actually takes

The figures usually quoted in the trade are that a lead-acid battery loses somewhere around 20% of its cranking output at freezing, and roughly half of it at around −18°C. Birmingham rarely sees the second number, but it sees the first several times a winter.

Meanwhile the oil is going the other way. This is exactly what the “W” in an oil grade means — 5W-30 is a winter rating of 5 and an operating rating of 30. A lower W number flows better when cold, which is precisely the cold-start problem: thinner oil means less drag on the starter and faster oil pressure once it fires. If your car is on a heavier winter grade than the manufacturer specifies, cold mornings are harder than they need to be. Getting the specification right matters more in January than in July.

Add a third factor people forget: a battery that has spent the autumn on short journeys never fully recharges, so it enters winter already down on charge. The deficit and the cold arrive together.

Diesel-only causes

Glow plugs. A diesel ignites by compression alone, and a cold engine loses too much heat to the cylinder walls for that to work reliably. Glow plugs pre-heat the combustion chamber to bridge the gap, then usually continue running for a period after start-up to reduce smoke and roughness.

The symptoms of failed plugs are distinctive because they are cold-only:

• Extended cranking on a cold morning, normal starting once warm
• White or grey smoke for the first thirty seconds
• Rough, lumpy running that clears as the engine warms
• The glow plug light staying on longer than usual, or flashing

On a four-cylinder engine one failed plug is often survivable and two generally is not. They are usually replaced as a set, and seized plugs are what makes the job occasionally expensive rather than the parts.

Fuel waxing. Diesel contains paraffin, which begins crystallising out as the temperature drops — the point at which those crystals block a filter is called the cold filter plugging point. UK fuel is blended to a winter specification through the colder months, which is why this is rare. It is not impossible, though, and the two situations that produce it are a car that filled up in autumn and then sat outside through a hard frost, or fuel bought abroad.

The signature is different from a battery problem: the engine cranks perfectly well, may fire briefly, and then dies — because the starter has plenty of power and the fuel simply is not arriving.

Water in the fuel filter freezing. Diesel filters separate out water, and some have a drain for exactly that. Left too long, that water can freeze and block the filter.

Petrol-only causes

Fouled spark plugs. A cold engine runs a deliberately rich mixture, and a car doing only short journeys never burns that deposit off. The plugs accumulate carbon over months, then fail to spark reliably on the one morning they are asked to do the hardest work.

Damp ignition components, on older cars with distributor caps and separate HT leads. Condensation tracks the spark to earth instead of the plug. Modern coil-on-plug systems are largely immune.

Flooding. Repeated failed attempts push unburnt fuel into the cylinders and wet the plugs, at which point more cranking makes it worse rather than better.

Three things that are not the engine at all

• The clutch or brake interlock. Most modern manuals will not crank unless the clutch is fully depressed, and most automatics need the brake. A stiff, cold pedal and a foot that is a centimetre short is a surprisingly common “won’t start” — and it costs nothing to rule out
• The key fob battery. Coin cells lose voltage in the cold. If the car does not recognise the key, most makes have a backup — usually holding the fob against a marked spot on the steering column or the start button
• A frozen handbrake. Not a starting problem, but the same morning and the same cause. Cables and shoes freeze after a wet day followed by a hard frost, and forcing it can snap a cable. Rocking gently in gear usually frees it

What to do, in order

1. Switch off every electrical load — headlights, heater blower, heated rear screen, heated seats, radio, phone charger. The heated screen alone is one of the largest draws on the car after the starter
2. Dip the clutch fully and hold it. This satisfies the interlock and it also disconnects the gearbox, so the starter is not turning cold gearbox oil as well as the engine
3. On a diesel, wait for the glow plug light to go out. On a very cold morning it is worth cycling the ignition on and off twice so the plugs get two heating periods before you crank
4. Crank in bursts of no more than about ten seconds, then wait thirty seconds before trying again. The pause lets the battery recover some voltage and lets the starter cool
5. Stop after four or five attempts. Beyond that you are flattening a marginal battery and, on petrol, risking flooding it

What happens on those attempts tells you which problem you have. Slow, laboured, dying cranking is electrical. Brisk, healthy cranking with no firing is fuel or spark. Fires and immediately dies is fuel. Nothing at all, or a single click, is a different tree again — the general no-start diagnosis covers what each of those sounds means at any time of year.

Jump starting, and the bit afterwards

Jump starting works and is worth doing properly. Connect positive to positive, then the donor’s negative to a solid unpainted earth point on the dead car rather than to its negative terminal — a battery vents hydrogen, and the last connection is where a spark occurs. Let the donor run for a few minutes before trying.

The part people get wrong is what happens next. A jump start puts almost nothing back into the battery. Driving five miles to work puts very little more in, because the alternator is also running the lights, heater and heated screen on a winter morning. So the car goes flat again the following day and everybody concludes the battery is dead.

It might be. But a flat battery needs either a sustained run of half an hour or more, or a proper charge from a mains charger overnight, before you can judge it. And if the car starts on leads and then dies within minutes once they come off, that is not the battery at all — that is the charging system.

Preventing it

• Have the battery tested in October. State of health, not “it started today”
• One sustained run a week if your driving is short local journeys. Twenty minutes or more at road speed genuinely recharges it where a commute does not
• Correct oil grade, particularly the W number
• Diesel: treat extended cold cranking as a glow plug warning, not a quirk. They fail one at a time and the car gets progressively worse over a couple of winters
• Keep the fuel tank fuller in winter. Less air space means less condensation, which means less water in the fuel

What does it cost?

Glow plugs sit at the upper end when they seize in the head, which is common on a plug that has been in place for years. It is one of the arguments for replacing them at the first symptom rather than the third winter.

Why it happens here

Birmingham does not get severe cold. It gets something that is arguably worse for batteries: a long, damp, marginal winter combined with a driving pattern that never recharges anything.

Across the terraced inner suburbs most cars sleep at the kerb, cold-soaking overnight with no shelter. Journeys are short, so the alternator never repays what each start costs. Traffic is heavy, so on a wet dark evening the car is running headlights, wipers, blower and heated screen at low engine speed, which is when the alternator produces least.

The result is entirely predictable and it is why batteries fail across the city in the same fortnight each December rather than spread evenly through the year. It is also why the cheapest fix is a diary reminder in October rather than anything mechanical.

When should you call a garage?

Do not keep cranking. If it has not started in four or five attempts, further attempts are spending battery you will need and, on petrol, risking a flooded engine.

Book a test rather than a battery if the car is cranking slowly. A slow crank can be the battery, the charging system, corroded terminals or a poor earth, and only one of those is fixed by buying a battery.

Book it this week if you are a diesel driver and cold starting has got noticeably worse over the last two winters. That is glow plugs failing one at a time, and they get harder to remove the longer they stay in.

Ring rather than drive in if the engine cranks strongly, fires and dies. That is fuel, and repeated attempts will not help.

When you ring, tell us exactly what the car does when you turn the key, whether it is only ever a problem from cold, and how long the journey to work is. Those three usually name the cause before the car arrives. Battery testing and replacement in Birmingham, seven days a week — and we test the charging system alongside it, because replacing one and guessing at the other is how the problem comes back in a fortnight.