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Faulty lights or indicators

A headlight that no longer lights, a light that fades at idle, indicators that won't come on or flash too fast: lighting is a safety and roadworthiness item. Most faults are electrical and accessible: a blown bulb, a blown fuse, a dead indicator relay (flasher unit), a corroded earth or terminal that cuts contact. A light that fades mainly at engine idle points rather to a weak charge (alternator/regulator) or a tired battery. Before any heavy dismantling, check the bulb, then the corresponding fuse, then the connections: it's often a matter of a few minutes. Beware of LEDs: an indicator that 'hyper-flashes' after switching to LED bulbs is due to a need for a resistor or a suitable relay, not a fault.

Possible causes: faulty lights or indicators

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

  1. Blown bulb

    Basic

    The most frequent and simplest cause: the bulb's filament has failed (impact, vibration, end of life), or the bulb is blackened. A single light affected (one headlight, one indicator, the stop) almost always points to the bulb. Confirm it by removing the bulb: a broken filament or blackened glass = to be replaced with a bulb of the same type and wattage (base and wattage engraved on it or in the manual). Don't touch the glass of halogen bulbs with bare hands (the grease shortens their life). Trivial cost, a few minutes to replace.

  2. Blown fuse

    Basic

    Each circuit is protected by a fuse that blows in the event of overcurrent or a short circuit. If a whole circuit goes out at once (for example all the lights, or the dash + the lighting), suspect the fuse: the fuse box is often under the seat or a side panel, with a diagram on the cover. A blown fuse is visible (broken strip) and is replaced with a fuse of the same rating — never a stronger one. Important: a fuse that blows again immediately signals a short circuit to be traced, not a fuse to upsize. Always keep a few spare fuses.

  3. Dead indicator relay (flasher unit)

    Basic

    The indicators are timed by a small flasher unit (relay). When it weakens, the indicators no longer light, stay on solid, or flash far too fast/slow. An abnormally fast rhythm on one side often indicates a blown bulb on that side (the unit 'sees' less load); a fast rhythm everywhere after fitting LEDs comes from a unit not suited to LEDs. Replacing the unit with a compatible new one (thermal original, or electronic/LED if you switch to LED) solves most cases. A cheap part, simple to replace (often a small plug-in box).

  4. Corroded earth or terminal

    Intermediate

    Lighting that flickers, fades with vibration, or works intermittently often betrays a bad contact: a corroded terminal, a poorly seated connector, or above all a corroded earth (the negative return to the battery via the frame). Damp and salt attack these points. Spot them by moving the connectors with the light on (the lighting reacts) and by checking the earths. Cleaning the contacts, applying dielectric grease and re-tightening the earths restores crisp lighting. It's free or nearly, and often overlooked.

  5. Weak charge (alternator/regulator/battery)

    Intermediate

    If the lighting fades clearly at idle and brightens when you raise the revs, the circuit voltage is low: the battery is tired, or the charging system (alternator/regulator) isn't keeping up. It isn't a bulb fault but a supply one. Confirm it with a voltmeter: at idle then at 3,000-4,000 rpm, the voltage at the battery terminals should rise toward ~13.5-14.5 V; if it stays low, the charge is at fault. See also the 'battery going flat' symptom.