Why cars break down in January and what to do about it in October

Winter does not really break cars. It finds the parts that were already marginal and tests them all on the same morning, usually the one where you are late. The battery that was fine in September, the wiper blade that smeared a bit, the tire that was down to its last couple of millimeters.

An afternoon in October is cheaper than one call-out in January, and most of it is genuinely easy.

Start with the battery

Cold works against a battery twice over. The chemistry slows down, so less current is available, and at the same time the oil in the engine thickens, so more current is needed to turn it over. Two curves move in opposite directions and meet on the coldest morning of the year.

The warnings are quiet. A crank that sounds slower than it did in summer. Headlights that dim noticeably while starting. Needing a second try after a cold night. If you have noticed any of those, get the battery tested — most shops and parts counters do it free, and what you want is a load or conductance test rather than a voltage reading, because a tired battery can show a healthy voltage right up until it is asked to do something.

While you are there: terminals clean and tight, and the hold-down bracket actually holding it down. One thing not to improvise on is the replacement type. Cars with stop-start use AGM or EFB batteries, and many of them need the new battery registered to the charging system electronically. Fitting a conventional battery instead, or skipping the registration, shortens its life considerably. That job belongs with someone who has the tool.

If your driving is mostly short trips, the alternator never fully replaces what starting takes out. A few hours on a mains charger once a month through winter does more for battery life than anything else you can do.

Tires, because everything else depends on them

Rubber compounds stiffen as they get colder, and a stiffer tire has less grip. The figure the tire industry generally works to is around 7 degrees Celsius, roughly 45 Fahrenheit — below that, a summer compound is working outside its best range even on dry pavement. Winter tires use compounds that stay pliable in the cold, plus fine slits across the tread blocks that bite into snow.

Which category suits you is a question about your climate, not your car. Mild, wet, rarely freezing: a good all-season is a reasonable compromise. Regular snow, ice, or long cold spells: proper winter tires are a different level of capability, and in some regions a legal requirement, usually specified by the three-peak mountain snowflake symbol rather than the older M+S marking.

Fit them in fours. Putting winter tires on the driven axle only leaves the other end with much less grip, and a car that grips at one end and not the other behaves in ways most drivers have never practiced. This is not a place to economize.

Pressure falls as the air gets colder, so a tire set correctly in warm weather will read low by midwinter. Check monthly through the cold months, always before driving while the tires are cold, against the placard in the door jamb. And treat the legal minimum tread depth as what it is: a legal minimum, not a recommendation for a snowy hill.

Seeing out

• Replace wiper blades before winter rather than after a month of scraping them across ice
• Fill the washer bottle with fluid rated below the coldest temperature you get — topping up with plain water can freeze the reservoir, the lines and the pump
• Clean the inside of the glass properly; the invisible film on the inside is what fog condenses onto first
• Check every exterior lamp and wash the lenses, including the reversing lights nobody looks at
• Run the air conditioning even in winter — it dries the air, so the screen clears faster

Two things not to do on a frozen morning. Do not pour hot water on cold glass. Do not switch the wipers on to clear ice before you have freed the blades, because the rubber edge tears off in strips and you will not notice until the first rain. Clear the whole car, not a porthole: the roof, the lights, the mirrors and the rear glass. A slab of snow leaving your roof at speed is somebody else's windshield.

Fluids and a look under the hood

Coolant is not just about boiling. The mix protects against freezing too, and a diluted system can freeze hard enough to crack expensive parts. You cannot judge concentration by color or by looking at it; a shop can test the freeze point with a refractometer in about a minute, which is worth asking for at the service before winter.

Engine oil matters in the cold as well. The number before the W in the grade describes how the oil flows at low temperature, which is exactly what a cold start depends on. Use the grade your handbook specifies for your climate rather than whatever is on offer. While the hood is up, look for perished or bulging hoses and for crusty residue around the water pump and hose joints — small leaks become roadside failures faster in winter.

What lives in the trunk

• An ice scraper and a snow brush, plus gloves that live in the car and never leave it
• A warm jacket and a hat, because the coat you drive in is not the coat you want to stand in
• A flashlight that works, and a charged power bank for the phone
• A blanket, water and something to eat that will not be ruined by a year of temperature swings
• Jumper cables or a lithium jump pack, and a folding shovel where snow is a real possibility
• A high-visibility vest and warning triangle, which are legally required in many countries

The reason for the jacket and the blanket is not the breakdown itself. It is the ninety minutes you spend waiting for recovery on a shoulder in the dark, with the engine off.

Driving habits beat equipment

Leave earlier and do everything more gently. Brake, then steer, then accelerate — one input at a time, because a tire that is being asked to do two things at once on a slick surface will usually do neither. Leave far more space than feels necessary. Avoid cruise control on anything slippery, since the system will happily apply power at the wrong moment. Keep the fuel tank fuller than usual, both for the range and for the hours you might spend idling to keep warm.

And if something already feels wrong — brakes pulling to one side, the steering wandering, a cold engine that will not idle cleanly — get it looked at now rather than in February. Everything marginal gets worse when it is cold, wet and dark.

Book the battery test in the same week you put the snow brush in the trunk. Both take ten minutes, and only one of them is any fun.