Why won't my car start but the radio and lights work?

Thu, 17/Nov/2022

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It sounds contradictory: the radio and interior lights work, so the battery must be fine — yet the engine will not start. In practice, starting needs a large burst of current (often 150–300 amps), while lights and radio draw far less. A battery can show 12+ volts at rest yet collapse when the starter loads the circuit.

Most common causes (in order)
1. Battery weak under load — the number-one cause. Dashboard lights stay on, but voltage drops too low to spin the starter. Try the headlight test: with headlights on high beam, attempt to start. Significant dimming points to the battery.

2. Starter motor or solenoid fault — especially if you hear one solid click and no crank, or intermittent no-crank days. Tapping the starter is a temporary old trick; proper fix is replacement or circuit repair.

3. Immobiliser not authorising start — key symbol or padlock light flashing on the dash, or engine cranks but will not fire. Try the spare key, replace the key-fob battery, and ensure the key is not near other electronics. After a flat battery or jump start, some cars need ignition left on for 10–15 minutes to relearn the key.

4. Ignition switch or starter relay — key turns but nothing happens, or no click at all despite good lights. A relay swap test (horn relay, if identical) is a quick DIY check on some models.

5. Park/neutral or clutch safety switch — automatics must be firmly in Park; manuals need the clutch fully depressed. Wiggle the selector and try again.

Cranks but won't run?
If the engine turns over normally but does not fire, the battery and starter are likely fine. Focus on fuel (listen for fuel pump prime when ignition is on), immobiliser warnings, and whether the fault started after running out of fuel or using the wrong key.

What a jump start tells you
If a jump start makes the car crank and run, plan a battery test the same week. If jump starting does not help and you still have one click or silence, suspect starter, immobiliser or wiring — book diagnostics rather than repeatedly cranking.

For roadside jump-start help: 020 8088 4023 or get a quote.