When a xenon headlight flickers, stays off, or flashes once and dies, drivers often replace the bulb first — but a failed ballast produces the same symptoms. The fastest route to the correct replacement is reading the part number from the control unit already in your car. This guide is a practical identification workflow; for D1S/D2S families and LED drivers, see our full ballast guide.

Rule out the bulb first

Swap the xenon bulb to the working side (same D-type). If the fault follows the bulb, replace the bulb. If the problem stays on the same headlamp, the ballast or wiring on that side is suspect. Never touch xenon bulbs with bare fingers — oil causes hot spots.

One side vs both sides

• One side only — usually one ballast or local connector; read that unit’s label.
• Both sides failed together — check fuses and lighting control module before buying two ballasts.
• Intermittent in wet weather — connector moisture or failing ballast; label still identifies the replacement type.

Where the ballast lives

On most cars the ballast is behind the headlamp: inside the wing, under the cover, or mounted on the back of the lamp unit. You may need to remove the headlamp or inner arch liner for access. Note left vs right only if the listing is side-specific — many units are identical both sides.

Reading the label

Photograph the whole label in good light. Record:

• Manufacturer name (Valeo, Hella, Osram, Mitsubishi, etc.)
• Primary part number (often 10–11 digits)
• OEM cross-reference (Audi/VW 8K0…, BMW 6311…, Mercedes A…)
• Bulb type hint (D1S, D2S, D3S, D4S) if printed

Search our headlight module category using that exact number — the same ballast often fits multiple brands under different OE prefixes.

Common manufacturer codes

Examples seen on UK cars (always verify against your label):

• Valeo 6G / 89034934 — used on Audi Q7, BMW 1 Series, Volvo, and others
• Hella 5DV / 5DC series — Mercedes, BMW, VAG
• Osram/A71177… — various Ford, Hyundai, Land Rover HID systems

A listing may show one car model for SEO but list compatible OE numbers in the description — match numbers, not badge alone.

Connector check

Replacement units must use the same plug shape and pin count. Compare photos of the connector on your old unit to the listing. Adapters exist for some retrofits but are not standard on OE replacements.

Search and order

1. Remove or access the faulty ballast and photograph the label.
2. Search our shop with the part number from the label.
3. Confirm HID/xenon system — halogen and full LED headlamps use different parts.

Full ballast reference

For xenon bulb families, LED driver modules, UK vs import headlamps, and coding notes, read our complete guide: Headlight ballast modules explained.

FAQ

Can I use any 35 W xenon ballast?

No. Wattage, bulb type (D1S vs D2S etc.), and connector must match. See the full guide for bulb families.

The label is unreadable — what now?

Note car make, model, year, and headlamp type (xenon adaptive or standard). Message us with photos of the unit and connector.

Should I replace in pairs?

Not required, but if one failed from age, the other may follow. Pairs make sense when labour to open the lamp is high.

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