If a car has trouble starting in cold weather, which is the LEAST likely cause?

Cold-start problems can come from many sources, but A/C failure is NOT among them — A/C is for cooling the cabin, has nothing to do with starting. Cold weather affects starting in several ways: (1) BATTERY CHEMISTRY slows in cold — chemical reactions producing current are temperature-dependent; at 0°F, a battery has only 60% of its 80°F cranking power; (2) ENGINE OIL THICKENS — viscosity rises in cold; thicker oil is harder to pump and creates more friction in bearings, requiring more cranking effort; multi-grade oils (5W-30) help — the 'W' rating is the cold viscosity; lower numbers flow better cold; (3) FUEL VOLATILITY DECREASES — gasoline doesn't vaporize as readily when cold; harder to ignite; modern winter-blend gasoline has more volatile components added; (4) STARTER MOTOR STRAINED — turning thick oil takes more current at the moment battery is weakest; (5) ELECTRICAL DEMANDS UP — heated seats, defrosters, heaters, lights all on at startup, drawing current that could go to starting; (6) DIESEL CONCERNS — diesel fuel gels in extreme cold (waxy components solidify); winter-blend diesel has anti-gel additives; very cold weather requires fuel heaters or block heaters; (7) GLOW PLUGS — diesels use glow plugs to preheat combustion chamber; weak glow plugs prevent cold start. Cold weather starting tips: (1) Use BLOCK HEATER (electric heater that warms engine block, oil pan, or coolant) for prolonged cold; common in northern climates; plug in for a few hours before starting; (2) DON'T pump accelerator (older carbureted cars only) — modern fuel-injected cars don't need this; (3) Keep BATTERY in good condition — replace before getting very weak; test annually; (4) Use correct OIL VISCOSITY for climate (manufacturer's recommendation accounts for typical climate); some recommend SAE 0W or 5W oils for very cold climates; (5) USE WINTER-GRADE FUEL — automatic for most US stations in winter, but watch in transitional weather; (6) KEEP FUEL TANK MORE FULL — reduces moisture condensation that can freeze in fuel lines; (7) PARK INSIDE if possible — even unheated garage is warmer than outside; (8) GIVE IT a moment — modern cars start almost immediately, but very cold cars may take a few extra seconds of cranking; alternate cranking 5-10 seconds then waiting 30 seconds (don't damage starter from continuous cranking). Battery age: typical lifespan 3-5 years in mild climate, 2-4 years in extreme climates. Replace before failure.

Jump starting: with cables and a running car (or jump-starter); (1) Connect RED to dead battery POSITIVE; (2) RED to good battery POSITIVE; (3) BLACK to good battery NEGATIVE; (4) BLACK to a GROUND on dead car (engine block, frame — NOT to dead battery negative; prevents spark near battery hydrogen gas); (5) Let donor run a few minutes; (6) Try to start dead car; (7) Disconnect in REVERSE order. Modern jump starter packs are safer (built-in protection). Don't try to push-start a vehicle with automatic transmission or a modern fuel-injected engine — won't work.