Why Do My Headlight Bulbs Keep Blowing?!


Hold on a Minute

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 just removing the front bumper and 100,000 of those dreaded, rusty bottom engine cover bolts just to swap out some pesky little light bulbs!

Reason #1 – Faulty Voltage / Alternator Regulator (A.K.A Voltage Stabiliser)

The voltage regulator is a critical piece of electrical equipment in a vehicle, they can usually be found attached to the side of the alternator 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’s most definitely the source of your bulb problems. This is 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’ll 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. This is a common ‘online forum’ notable fix for some vehicles. Try replacing the regulator first to see if it fixes the problem.

Reason #2 – Loose wiring connections to the bulb holder/bulb

This can cause the current or ‘flow’ of electricity to contact intermittently on and off, this can in-turn 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

Reason #3 – Poor Quality Bulbs

This one might sound like a bit of a cliche, 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.


Reason #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.

Dirty Hands
Touching bulbs with hands this oily is a recipe for premature failure

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.

Reason #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.


Reason #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.

Headlight Condensation

Aftermarket Headlights & Condensation

Some after-market ‘copy’ headlights are made so poorly that a large amount of condensation can be seen after just a couple weeks. It’s worth noting that minor condensation is perfectly normal, however when ‘pools’ of water form at the bottom corners it should be checked out ASAP. The staff here have their own experiences with after-market headlights and can safely say they’re a waste of time. For this reason among others, we only sell original (OEM) branded headlights due to wanting the very best for our customers.

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,

      Because you said you replaced the bulbs then started having problems, the first thing I’d check is the connections to the bulbs are properly connected and home all the way. Then check that the input voltage being supplied to the headlight isn’t too high by performing a voltage regulator check as described in the article. Also, check for partial shorts (high resistance).

      To check for shorts, you can either buy a short checking tool or set your multimeter to Ohms (symbol: Ω) and do the following. To check the wiring from the fuse that keeps blowing to the switch on the dash, disconnect the bulb and measure between the fuses terminal and ground, the resistance should be infinite (0.00), the engine doesn’t have to be running for this.

      Refit the fuse and then take a reading again but from the bulbs connector to ground. If it reads anything other than infinite, you know there’s a wiring short or partial short on this section of the circuit instead of the fuse box to switch (perhaps a damaged wire rubbing against the body). Another sign of a high resistance short is that the bulb on one side may not be as bright as the other side.

      Hope this helps

  • My Clio 3 dim globes fail periodically, but I never had to replace the bright globes. The deteriorated plastic connector block gave a clue to what is happening – heat buildup, and it is definately not bad connectors. I’ve discovered the same phenominon on the connector block for the heater blower resistors. The resistors are inside the airflow of the fan . I’ve drilled a small hole in the connector block to deliver air to the connector block, and it solved the problem. What to do to the badly designed dim light ventilation still has to be tackeled.

    • Have you checked you’re running standard wattage bulbs? Higher wattage bulbs increase heat build-up in the wiring loom as the gauge needs to be thicker. Sounds like too much voltage or current is being put through the system, have you performed a regulator check?

  • Cheap headlight globes often fail because the wire end of the filament where it connects to the connector block tag is poorly “crimped”, resulting in a bad connection causing heat buildup or entire disconnection of the current. Also the wattage rating of the bulb should not exceed the manufacturers specification. Where a 90 Watt lamp is fitted instead of a the usual 55Watt, be prepared for failure due to heat buildup and melting connector blocks made of plastic. Bakelite connector blocks handle heat better than plastic ones.

  • I have a 2009 Silverado dually and the headlights are halogen and the back of the headlight we’re it plugs into the wiring harness and the plug-in that goes into the headlight they both show signs of heat the Plastics melted I call that poor engineering regardless of whatever the problem is.
    I think the engineers who build these cars they never work on them so they have no idea what’s really wrong with them and I had to take the air cleaner box off just to get access to the halogen headlights and then you can’t get any parts for a truck that old.
    And I also think the Tariffs should be put on poor quality Parts especially from China.

    Article Rating: 5
  • Loose wiring connections was the problem in my case. As soon as I got it fixed, everything started to work fine. Appreciate your article!

  • Hi! I’m planning to replace my car’s halogen bulbs. Looking at my owner’s manual my headlights are composed of 4 bulbs (2 on each headlight). For low beam bulb the wattage is 51 but, can i replace it with a 55w halogen bulb? The high beam bulb is 60w, can i replace it with a 65w bulb? I’m worried that a 4w and a 5w difference respectibely can cause problems with the wirings or sockets. Thanks!

    • Hello,

      Automotive bulbs aren’t simply based on bulb wattages, there are multiple different bulb types that all have different sockets, beam patterns and designs.

      If the current bulbs are 51W (most likely a HB4 bulb which is 51W) then you will want to stick to 51W bulbs. It sounds like you may be trying to fit a different bulb type that has a higher wattage such as H7 which are 55w.

      Best to stick to the default bulb designation. You can use the OSRAM bulb finder website to confirm fitment and upgrade options.

      For example,

  • Hello, thanks for the article!
    My old BMW has this problem: one xenon lamp stopped working. Bulb itself was good, so at first I thought it was the ballast block and changed it, but nothing happened. Then I measured voltage that goes to the block and it turned out to be 29-30V which is too much (has to be ~13V). What do I do now? There’s a short circuit somewhere or..? Guess I have to check the V on the input to the headlight, there’s a connector with like 10 wires in it and I got no idea which one is the +…

    • Hello Alex,

      This points towards a faulty voltage regulator. We’d advise getting this checked at your local garage.

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