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Start with the symptom, not the parts

When a car refuses to start, the temptation is to dive straight for the battery or ring a garage. Resist that urge for five minutes. The single most useful question you can ask is what exactly happens when you turn the key or press the button. That answer narrows the fault down faster than any amount of guessing.

Broadly, a non-starting car falls into one of three camps. In the first, you turn the key and hear nothing at all, or just a rapid clicking. In the second, the engine turns over normally, cranking away enthusiastically, but never fires. In the third, it fires briefly and dies. Each points to a different system, and the tests below follow that order.

Before you touch anything, make sure you are parked safely with the handbrake on, the gearbox in neutral or park, and the ignition switched off. If you are checking anything under the bonnet, let the engine bay cool first, and never lean over a battery while wearing loose clothing or a dangling scarf.

Silence or clicking: the battery and starting circuit

A flat battery is comfortably the most common cause of a car that will not start, and it is also the easiest to rule out. Switch on the headlights. If they glow dimly or not at all, the battery is discharged or the main earth is poor. If they are bright but everything dies the moment you turn the key, you are looking at a battery that has enough surface charge to light a bulb but not enough muscle to crank an engine.

If you have a multimeter, a healthy lead-acid battery should read around 12.4 to 12.7 volts at rest, and should not drop much below 9.6 volts while cranking. Anything under 12 volts at rest suggests the battery wants charging or replacing.

• Check the terminals for white or blue-green corrosion, and for anything loose enough to twist by hand.
• Look for a battery that is over five years old; capacity fades quietly with age, especially through a cold British winter.
• Test the alternator later, not now. A car that starts happily with jump leads but dies again after twenty minutes of driving is telling you the alternator is not charging.
• Be aware that short town journeys, heated seats, lights and a stop-start system are a punishing combination for a small battery.

If jump starting brings it to life, you have your answer. If jump leads make no difference at all, move on.

One loud click, then nothing: the starter motor

If the battery is strong, the lights stay bright when you crank, and you hear a single heavy clunk rather than the engine turning, the starter motor or its solenoid is the prime suspect. The solenoid is the switch that throws the starter pinion into the flywheel; when it fails, you get the click without the turning.

A few practical checks. First, look at the thick cable running from the battery to the starter and confirm both ends are tight and free from corrosion. Second, listen for a rapid chatter of clicks, which usually means the battery is too weak to hold the solenoid in, not that the starter is broken. Third, if you can safely reach the starter with the ignition off, give the body a firm tap with a wooden handle or spanner while a helper turns the key. If it suddenly cranks, the brushes or solenoid contacts are worn and replacement is only a matter of time. On many modern front-wheel-drive cars the starter is buried behind the engine, so this is often a job best left to a garage with a ramp.

Turning over but never firing: fuel delivery

Cranking that sounds perfectly healthy but never catches usually means the engine is missing fuel or spark. Start with fuel, because it is easier to test.

Turn the ignition on, without cranking, and listen. Most petrol cars prime the fuel pump for two or three seconds, producing a soft whine from under the rear seats. Silence there is a strong clue. Now check the fuel gauge properly. It is remarkable how often a faulty gauge or a car parked nose-up on a steep driveway leaves a driver convinced there is half a tank when there is not.

• On a diesel, a blocked fuel filter or air drawn into the system after running dry can prevent starting entirely.
• Many petrol cars have an inertia fuel cut-off switch, designed to trip in a collision. Its location varies, but the handbook will tell you where it is and how to reset it.
• Check for a strong smell of petrol around the engine bay, which points to a leak rather than a supply problem.
• Do not crank endlessly on a car that may have a flooded engine. Floor the accelerator once and crank briefly on many older petrol models, then give the starter a rest.

Spark, sensors and the immobiliser

If fuel is arriving, look at ignition. Worn spark plugs, tired coil packs and damp ignition components cause rough running, misfires and, eventually, a refusal to start. A car that will not start on a wet morning but behaves perfectly by the afternoon is practically advertising an ignition fault.

Do not overlook the humble key. A flashing immobiliser light on the dashboard while the engine cranks means the car has not recognised the key, often after a flat key-fob battery or a knock. Try the spare key before assuming anything expensive. Crankshaft and camshaft sensors, coolant temperature sensors and, on some models, a failing fuel pump relay can all cut the show without warning, and these usually leave a fault code behind. A cheap code reader plugged into the diagnostic socket will often point you in the right direction for the price of a takeaway.

When to stop and call for help

Work through the systems in order: battery, starting circuit, starter, fuel, then ignition. That sequence solves the large majority of no-start faults and saves you from buying parts you do not need.

Stop and seek professional help if you smell petrol strongly, see smoke, find bare or chafed wiring, or if the car is in an unsafe position. Be honest about your limits too. Diagnosis on a modern car increasingly means live data and wiring diagrams, and a good local garage will thank you for describing the symptom precisely rather than guessing at the cause. A clear account of what you hear, smell and see is worth more than any amount of tinkering.