Why Do My Headlight Bulbs Keep Blowing?

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Most of us with vehicle ownership experience will know that changing light bulbs on a vehicle should be as easy as just removing the cover from the back of the light, and slotting a new one straight in; easy peasy right? Unfortunately not, some vehicles can become your nemesis when it comes to bulb changes. Who would of thought that a very few specially selected vehicle designers have a unique ambition in life to make headlight bulb changes one of the most frustrating, time consuming, expensive, hand-torturing maintenance process there possibly is to do on a car?

The point is proven when last week you spent most of Sunday morning removing the front bumper and 100,000 of those dreaded, rusty bottom engine cover bolts just to swap out some pesky little light bulbs!

So what are the most common reasons bulbs just keep blowing?

1. Faulty Voltage / Alternator Regulator (A.K.A Voltage Stabilizer)

The voltage regulator is a critical piece of electrical equipment in a vehicle, they can usually be found attached to the side of alternators or mounted separately. In easy-to-understand terms, it limits the maximum amount of voltage that runs through the electrical system to a safe and usable amount for all components to utilise.

If the voltage regulator is intermittently having problems then it could definitely be the source of your bulb problems due to spikes of high voltage jumping through the system, which in-turn blow the bulb filaments and/or headlight fuses.


You can perform a simple test to check if the voltage regulator is faulty, however you will need another person to help you rev the engine whilst you take a multimeter reading.

  • Acquire a multimeter if you don’t have one, you can usually purchase one for under £10
  • Open the bonnet and remove the positive red terminal safety cover from the battery (if there is one)
  • Set your multimeter to 20 DC volts as shown in the photo. The DC Volts symbol is a V, and next to it there should be three dots with a straight line on top
  • Don’t use the symbol with a V and a squiggly line next to it, as that’s AC volts. Useful tip – think of the squiggly line as alternating current (AC) to remember this!
  • Before you begin testing, make sure the ignition and all of your vehicle’s lights are off.
  • Connect the red + (positive) multimeter test lead to your battery’s positive terminal.
  • Then, connect the black – (negative) multimeter test lead to your battery’s negative terminal.
  • Have your assistant turn on the engine (check it’s in neutral first)
  • The multimeter reading should rise to around 13.8 volts whilst the car is idling, this means the alternator is charging the battery correctly
  • Ask the person helping to slowly rev the engine until it reaches 1500 – 2000 RPM
  • Check the multimeter reading and jot it down, the voltage regulator should cap the amount to 14.5v. If the reading is over 14.5v then your voltage regulator is most definitely faulty.

Another known problem in the same category is that some poorly manufactured alternator belts can cause power surges due to static build up. This problem can be found by performing the check above and hopefully fixed by replacing the alternator belt with a premium brand or by installing resistors to the low and high beam circuits which is a common ‘online forum’ notable fix. The problem is that the surge may also be the voltage regulator, so try replacing the regulator first to see if it fixes the problem.

2. Loose wiring connections to the bulb holder/bulb

This can cause the current or ‘flow’ of electricity to arc between the connections on and off which in-turn can suddenly cause an increase in heat. Temperatures that go beyond what the bulb is designed to cater for will easily cause the filament inside to blow.

Vibrations in the headlight can also increase this problem, so check to make sure all headlight bolts are tight, that no headlight mounts are cracked and that the electrical connectors to the bulbs are secure. We recommend fitting new connectors to the bulb to completely eradicate them as the source of problems, especially if they look burnt, melted or show signs of corrosion inside.

H4 Plug Connector

3. Poor quality bulbs

This one might sound like a bit of a cliché, however the truth is that dirt cheap bulbs are no where near the quality standards of high-end brands and this why we will never sell them. The wire filaments are usually made from a far thinner gauge of tungsten which can see them fail in a matter of hours. Vibrations are the most common cause of failure on cheap bulbs, and this combined with thin gauge wire results in easy failures, especially if there are voltage fluctuations going on with a dodgy regulator. It’s an obvious sign of household technology put into a car bulb, so failures are bound to happen easily, have you ever seen the wire wobble around inside a household bulb? Stick to a high quality German brand such as Osram, they’re available in our shop.


4. Touching the bulbs glass on installation

Halogen bulbs obviously get very hot during operation and the chances are that by the time you get to the stage of actually fitting the bulb, your hands look like this. Bulbs need to heat evenly throughout the whole of the bulbs surface for optimum lifetime, leaving traces of dirt and grime or even the oil from your skin can sometimes cause uneven heating when the bulb is in use.

In some extreme cases, dirt on the bulbs glass can cause a complete structural failure and they end up exploding! Use clean latex gloves and only try to touch the bulbs metal base when installing, this can be very tricky when sometimes they’re hard to position correctly with limited amounts of space.

Dirty Hands

5. Excessive vibration

Vibration is not a good thing for halogen filaments, which is also another reason why Xenon HID bulbs excel on lifetime as they don’t have any! Check the bulb holders retaining spring is clipped in correctly and as said before, check the headlight is secured nice and tight. In extreme scenarios it may be worth checking the condition of wheel bearings, suspension springs and wheel balance, literally anything that can cause major vibrations at the front usually contribute to bulbs blowing.


6. Excessive condensation in the headlight

Too much condensation in headlights can cause electrical shorts and diminish the lifetime of bulbs rapidly, so it’s important to check if they’re getting too damp inside.

Headlights are usually ventilated through the top and bottom of the unit through small holes or tubes usually found with a 90 degree bend to them. They’re designed to only allow a certain amount of air to flow in and out for heat dissipation but also to help reduce moisture in the lens. If the bulb covers on the back of the headlight aren’t mounted correctly or any seals have broken around the lens, then you can expect a higher amount of condensation in the headlight than normal.

Some after-market ‘copy’ headlights are made so poorly that a large amount of condensation can be seen after just a couple weeks. The staff here have their own experiences with after-market headlights and can safely say that they’re a waste of time and are a catalyst for condensation. For this reason we only sell Original (OEM) branded headlights such as Hella, due to only wanting the very best for our customers.


Headlight Condensation


Have you had a really bad experience changing headlight bulbs? Let us know in the comments section below.

Article Rating: 5


  • you should write for Grumpy Old Men/Women!

    “who would of thought that a very few specially selected vehicle designers have a unique ambition in life to make headlight bulb changes one of the most frustrating, time consuming, expensive, hand-torturing maintenance process there possibly is to do on a car?

    This is even more relevant when you found yourself a week ago spending the whole Sunday removing the complete front bumper as well as the bottom engine cover that’s secured on by horrible rusty bolts, just so you could remove those pesky little headlights to replace some bulbs!”

    Haha, I feel your pain! Laughed a lot.

    Article Rating: 5
  • So my boyfriend’s right side headlight keeps blowing. He installed a new one less than a week ago and today that same one blew again. I think the frustration is that it’s happened FIVE TIMES NOW! This article is so so very helpful! Thank you!!!

    Article Rating: 5
  • Hi I replaced both of my headlight bulbs as they weren’t very good on my aldi a6
    After a day nearside headlight and indecater and side light all went dead
    Found a 30 amp fuse had blown in the boot replaced fuse and it went again took bulb back and got a replacement put another 30 amp fuse in and that went .So I replaced it with my old headlight bulb and the fuse went again cant under why as the offside is working fine with the new bulb

    went of

    • Hello Ed,

      The first thing to check is the input voltage that’s being supplied to the headlight under load conditions. Perform the voltage regulator check in the article and also check for partial shorts (high resistance). Fit a new fuse first and obviously check the headlight switch is turned on.
      Set your multimeter to 20 DC volts, the DC Volts symbol looks like a V with a thin ‘E’ on top of it sideways and check that while revving the voltage doesn’t go higher than 14.5v. If it does then this is the cause of your bulbs blowing as too much voltage is being supplied to the headlight. Note that voltage regulators can intermittently fail with spikes going through the system so it’s not always instant.

      To check for a short or partial short to ground, check that by disconnecting the bulb and measuring between the fuse terminal and ground, the resistance (Ohms and multimeter symbol: Ω) should be infinite (0.00). If it is infinite and all is okay, remove the headlight bulb from the headlight, refit the fuse and take the reading again from the bulbs connector to ground. If it reads anything other than infinite you know there’s a wiring short to ground inbetween the headlight and the fuse box (perhaps a cut wire against the body). Another sign of a high resistance short is that the bulb may not be as bright as the other side.

      Hope this helps

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