When your car won't start, the most useful clue is exactly what you hear and see when you turn the key or press the start button.
Symptoms and likely cause
- Nothing happens, dashboard lights are dim or off: flat battery or a poor terminal connection.
- Rapid clicking: the battery doesn't have enough current to turn the starter. Usually the battery.
- One loud click, lights stay bright: the starter motor or its solenoid/relay.
- The engine cranks normally but doesn't fire: fuel, ignition, immobiliser or a sensor (for example the crankshaft position sensor).
- Slow cranking in cold weather: a weak battery or oil that is too thick.
Checking the battery with a multimeter
- A healthy battery at rest: 12.6–12.7 V. Below 12.4 V it is half discharged, below 12.0 V it is flat.
- With the engine running you should see 13.8–14.7 V. Below about 13.2 V the alternator isn't charging properly.
- Inspect the terminals: white or green corrosion and loose clamps cause the same symptoms as a flat battery.
Jump-starting safely
Connect positive to positive first, then the donor car's negative to a bare metal point on your engine (not directly to the flat battery's negative terminal). Let the donor car run for a few minutes, then try. On newer cars check the owner's manual for the designated jump points. Once it starts, drive for at least 30–40 minutes or charge the battery with a charger.
Why does the battery keep going flat?
Short city trips, an old battery (typically 4–6 years), a parasitic drain (something that doesn't switch off), or a weak alternator. If the battery goes flat overnight more than once, the parasitic drain needs to be measured.
Starter or immobiliser?
If a key symbol flashes on the dashboard while cranking or nothing at all happens, the immobiliser may not recognise the key. Try the spare key before replacing parts – a flat key fob battery is a common and cheap culprit.
Describe your symptoms in Diagno's AI diagnosis for a first assessment. A trusted mechanic can test the battery, alternator and parasitic drain properly.