Why won’t my car start?

This single question saves more diagnostic time than anything else, and you can answer it yourself before calling anyone.

Does the engine turn over — that rhythmic churning sound — but never catch and run? Or do you get a click, rapid clicking, or complete silence with no churning at all? Those are two different problems with almost no overlap in causes.

This points at the electrical side. A single loud click is often the starter solenoid engaging while the battery lacks the current to spin the motor. Rapid machine-gun clicking is a classic low-battery signature.

Before assuming the battery, check the terminals. Corrosion or a loose clamp will produce identical symptoms to a dead battery, and cleaning and tightening a connection is free.

The battery and starter are doing their jobs, so the problem is one of the three things combustion requires: spark, fuel or compression.

Fuel problems are common. If you hear no brief hum from the fuel pump when the key first goes to on, that is worth mentioning. Spark problems often come with a stored crank or cam sensor code. Compression problems are less common but more serious, and usually announce themselves with a change in cranking sound.

One more thing worth checking: if a security or immobilizer light is flashing, the car may be blocking the start deliberately because it does not recognize the key.

An intermittent no-start is the most useful kind to diagnose, because the fault is still coming and going. Waiting until it fails permanently often makes it easier to find but leaves you stranded first.

Note the pattern and tell the shop: only when hot, only when cold, only after sitting overnight, only with a full tank. Those patterns point directly at specific components.