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Car Won't Start — Causes, Diagnosis and What to Do

Car Won't Start — Causes, Diagnosis and What to Do

A car that won’t start is one of the most frustrating situations a driver can face — especially when it happens on your driveway in the morning. The good news is that the majority of non-start faults have a small number of common causes that can be diagnosed quickly and fixed the same day.

Step 1: What Does It Do When You Turn the Key?

The sound and behaviour when you turn the key is the most useful diagnostic clue:

Nothing at all — no click, no crank

  • Battery completely flat or disconnected
  • Faulty ignition switch
  • Faulty neutral/clutch safety switch (automatic gearbox or depressed clutch required to start)
  • Blown main fuse

Rapid clicking

  • Battery flat or failing (insufficient cranking amps)
  • Poor earth cable connection
  • Corroded battery terminals

Single heavy clunk

  • Starter motor solenoid engaging but motor not spinning
  • Starter motor mechanical failure
  • Occasionally: engine seized (very rare — check oil first)

Engine cranks slowly but won’t fire

  • Battery weak but not totally flat
  • Engine fault: fuel, ignition or compression issue
  • Cold weather effect on battery and fuel viscosity

Engine cranks normally but won’t start (no fire)

  • No fuel: fuel pump failure, empty tank, blocked fuel filter
  • No spark: coil pack failure, crankshaft sensor fault (P0335)
  • Immobiliser active: key not recognised, faulty transponder chip
  • Flooded engine (rare on modern fuel-injected vehicles)

Diagnosing a No-Start at Home

Check the battery first

  1. Turn on the headlights. Are they bright or dim?
  2. Attempt to start. Do they dim further or go out?
  3. If they dim significantly or go out — the battery is the problem

Check for immobiliser

Look for a flashing immobiliser light on the dashboard. If the car has not recognised the key, it will crank but not fire. Try the spare key. If this is a new issue, the key transponder chip may have failed — a common fault on older keys.

Check for fuel

Turn the ignition to position II (not start) and listen at the fuel filler cap for the fuel pump priming. A brief hum lasting 2–3 seconds should be audible. No sound may indicate fuel pump failure.

Can I Fix It Myself?

Jump start — If the battery is flat due to leaving lights on or a one-off discharge, a jump start may recover it. After jump-starting, drive for at least 30 minutes to allow the alternator to charge the battery. If the battery goes flat again quickly, it needs replacing.

Battery terminals — Corroded or loose battery terminal clamps can cause starting faults. Clean terminals with a wire brush (disconnect negative first), reconnect firmly.

When to call a mechanic — If the battery is healthy but the car still won’t start, or if a jump start doesn’t help, the fault requires diagnostic testing. We carry a professional battery tester, starter circuit testing equipment and an OBD2 scanner to identify the root cause on-site.

How We Diagnose a Non-Start

  1. Battery voltage and health test (conductance test)
  2. Starter circuit voltage drop test
  3. OBD2 scan for fault codes (immobiliser, crankshaft sensor, fuel system)
  4. Fuel pressure test (if available and fault points to fuel system)
  5. Ignition system test (coil output, spark)

In most cases the fault is identified within 30 minutes and repaired the same visit.

Book a Non-Start Diagnosis in Reading

Call 07305 523333. We cover all Reading postcodes and surrounding Berkshire towns. For same-day attendance, call early in the morning or the evening before.

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Frequently asked questions

My car won't start but the lights work — what is the cause? +

If the interior lights and dashboard illuminate but the engine won't crank, the battery has enough power for low-current circuits but not enough cranking amps for the starter motor. This is very common with a weak or partially discharged battery. Try jump-starting with jump leads.

My car makes a clicking noise when I try to start it — what does that mean? +

A rapid clicking sound (relay clicking) means the battery does not have enough power to turn the starter motor. This is almost always a flat or failing battery. A single heavy clunk suggests the starter motor solenoid is engaging but the motor is not turning — the starter motor may need replacement.

Can a mobile mechanic fix a car that won't start? +

Yes. We attend to non-starting vehicles, carry out diagnosis on-site and in most cases carry the parts needed (battery, starter motor, ignition relay, fuel pump relay) to fix the fault the same day.