Sylvania says headlight bulbs dim over time and can lose up to 50 feet of down-road visibility before they burn out, and it recommends replacing them in pairs so both sides match. AutoZone’s battery guide notes that lights that dim and then brighten depending on engine speed suggest inconsistent voltage from the alternator.
Likely causes
Steady dimness and flicker usually have different causes.
| Cause | Clues | DIY or shop |
|---|---|---|
| Aging halogen bulbs | Both sides dim, yellowish light, bulbs several years old | DIY: replace both bulbs |
| Cloudy or yellowed lenses | Hazy plastic, weak and scattered beam | DIY restoration kit or new housings |
| Loose connector or bad ground | One side flickers, especially over bumps | DIY: check the connector and ground |
| Charging system problem | All lights dim at idle and brighten when revving; battery light | Free alternator test |
| Incompatible LED or HID retrofit | Flicker or warning lights after an upgrade | Use bulbs approved for your housing |
| Mis-aimed headlights | Short beam, glare complaints after a repair | Shop or DIY aim per the owner’s manual |
From Sylvania and AutoZone published guidance; not a diagnosis of your vehicle.
What you can check yourself
- Park facing a wall at night and compare both beams for brightness and color.
- Rev the engine gently in Park: lights that brighten noticeably suggest low charging voltage at idle.
- Check the bulb connector for melting, corrosion or looseness.
- Use the bulb size listed in your owner’s manual or Sylvania’s bulb finder.
When to go to a shop
If the bulbs are new, the connectors are tight and the charging system tests fine, a wiring or headlight module fault may need diagnosis. Some cars need the bumper or air box removed to reach the bulbs, which is when shop labor starts to make sense.
Safety: turn the lights off and let bulbs cool before touching them, and do not touch the glass of a halogen bulb with bare fingers. Sylvania recommends gloves or a clean towel.
Bulb prices
The Sylvania H11 bulbs we checked cost the same at AutoZone and Advance. Compare headlight bulb prices, and if the lights dim with engine speed, read battery light on while driving.
Frequently asked questions
Most often aging bulbs or cloudy lenses. If the dimming changes with engine speed, the charging system may be the cause.Why are my headlights dim?
Yes. Sylvania recommends pairs because bulbs age together, and a new bulb next to an old one gives uneven light.Should I replace headlights in pairs?
Common causes are a loose connector or ground, a failing bulb, an incompatible LED retrofit or low charging voltage.Why do my headlights flicker?
Keep comparing
Sources
We compare auto parts on published specs, warranty terms and dated store prices, and we link every store policy to its official page. We do not run a lab, so we never claim hands-on tests we did not do. About us · How we compare · Editorial policy