Few things are more frustrating than turning the key, or pressing the start button, and getting nothing, a click or a slow groan. The cause is usually one of three parts: the battery, the starter motor or the alternator. A few simple checks can tell you which is most likely before you spend money on parts.
How the starting system works
The battery supplies the power to start the car. The starter motor uses that power to turn the engine until it fires. Once the engine is running, the alternator recharges the battery and powers the car's electrics. A fault in any one of these can leave you stranded, and they often get blamed for each other's faults.
Step 1: What happens when you try to start?
- Nothing at all, no dashboard lights: a completely flat battery, a loose or corroded battery terminal, or a main fuse.
- Dashboard lights come on but dim or flicker when you try to start: usually a weak battery.
- A single click, lights stay bright: often a starter motor or starter solenoid fault, or a poor connection to the starter.
- Rapid clicking: usually a battery that doesn't have enough charge to turn the starter.
- Slow, laboured cranking: a weak battery, poor connections, or occasionally a tired starter motor.
- Normal cranking but no start: the problem is more likely to be fuel, ignition or an immobiliser fault.
Step 2: Check the battery
A simple multimeter is a useful tool. With the car off and rested for a few hours, a healthy 12-volt battery usually reads around 12.6 volts. Around 12.4 volts suggests it is partly discharged, and 12 volts or less suggests it is flat. A battery can show a good voltage but still fail under load, so a garage or parts shop battery test is the best check.
Also look at the terminals. White or green corrosion and loose clamps can stop current flowing even when the battery is good. Batteries usually last several years, but cold weather, short trips and long periods parked all shorten their life.
Step 3: Starter motor clues
If the battery tests well but the car only clicks, the starter is the main suspect. Other signs include:
- A grinding or whirring noise without the engine turning
- An intermittent fault where the car sometimes starts and sometimes doesn't
- Smoke or a burning smell near the starter after repeated attempts
Before replacing the starter, check the main cable and earth strap connections, which can cause the same symptoms.
Step 4: Alternator clues
The alternator is the likely culprit if the car starts after a jump but the battery goes flat again within a day or two. Watch for:
- A battery warning light while driving
- Dimming lights or electrical items behaving oddly
- A whining noise from the front of the engine
- A squealing auxiliary belt
With the engine running, the voltage at the battery is usually somewhere between about 13.8 and 14.7 volts. Much lower suggests the alternator isn't charging. Some modern cars use smart charging and can show lower readings at times, so a proper charging system test is the best confirmation. Also check the auxiliary belt and tensioner; our guide on auxiliary belt squeal explains why.
Stop-start cars need the right battery
Cars with stop-start systems use EFB or AGM batteries designed for frequent restarts. Fitting a standard battery can lead to early failure and stop-start faults. Many cars also need a new battery to be registered with the car's control unit so it can charge it correctly.
When a genuine starter or alternator is worth it
Starters and alternators work hard, and quality varies a lot. Genuine units are matched to the engine's power needs, mounting points and connectors. On cars with smart charging or stop-start, the alternator also communicates with the car's electronics, and the wrong unit can cause warnings or poor charging. A genuine part avoids those problems and is backed by the manufacturer's quality standards.
To find the right part number, use our guides to using your VIN and reading VW Group part numbers. Engine and gearbox type, and whether the car has stop-start, often change the part needed.
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