Diagnosis: If a car headlight doesn't work on low-beam...

If a car headlight works normally on high beam, but not on low-beam, is there a way to quickly/easily tell if the problem is:

–bad headlight bulb, requiring replacement
–bad fuse
–bad electrical connection

I assume I can save $$ doing my own diagnosis, but being a certified klutz, need to know the best approach to this…

Thx for info–

If only one side does not work it’s almost certainly a bad bulb.

If neither side works (but high beams work) it could be a lot of things. Likely the dimmer switch on the steering column but probably time to see a mechanic.

If you’re truly certified :smiley: you need to find and understand the replacement procedure before tearing stuff apart. Replacing headlights on some modern cars can be challenging and it’s possible to break stuff that’s not fixable.

For some reason, my low-beam headlights burn out far faster than I think they should. Anyway, this happens to me all the time, and it’s always just a burnt-out bulb. I don’t know for sure, but I would suspect that both bulbs are on the same circuit/wire/whatever. I’m a biologist, not an electrician.

The Car Talk guys addressed a similar question once. Someone claimed their low beams blew simultaneously all the time. The guys said actually since modern headlights are pretty bright, it’s more likely that only one blew out and the driver didn’t notice, until the second one blew.

Made sense to me.

Does the car only have one bulb for both high and low? That’s the way most cars are now. The bulb has two filaments, high beams and low beams, and the one for low has blown. You need a new bulb.

I’ve had the same problem. My way to diagnose the problem is to change the bulb and see if it works.


Question: do you replace them yourself? With halogen headlight bulbs you really, really need to make sure you don’t touch the bulb while you’re installing it, or wipe it down with rubbing alcohol if you do. The oils from your skin will heat up and drastically shorten the lifespan of the bulb. (It’s less of an issue for high beams because unless you live someplace super-rural you’re probably not leaving those on for hours on end.)

Can you not see the filament? You should be able to place an ohm meter across the build and see if there is resistance. OTOH how many cars use LEDs now?

My car has different bulbs for low and high beams. Recently I had both low beam bulbs burn out, and I had to use high beams to see at night. I got the bulbs replaces the next day but my high beam bulbs worked fine.

My informal survey of the parking lot I can see out the window shows that most of them are the the style that has separate bulbs. In a lot of countries that require DRL’s, they can’t be the same bulb as the low beams but they can use the high beams at reduced voltage, so at least among cars with big international markets that’s the most common arrangement.

Why go through the extra effort? To see the element or to do an ohm test you need to take the bulb out to begin with. At the point where i already have it out replacing a bulb is faster than pulling out my ohm meter.

My investigation always starts after replacing the bulbs. Bulbs are cheap, time is expensive. If the problem was the bulb no more time required.

A failed fuse is a possibility. Some vehicles use four separate fuses, one each for left low, left high, right low, right high. Other designs use less fuses for the headlights.

Burnt out bulbs are by far the most common cause; wiring problems are rare.

HID systems may have a couple other very expensive parts (module, igniter?) to go along with their very expensive bulbs. Testing those parts is not easy, but swapping bulbs is usually fairly simple and is a logical first step in diagnosis.

Back to non-HID systems, it does make sense to check fuses first, but it’s not a bad strategy to just put in a new bulb, as they’re pretty cheap and will fix the problem 90+% of the time. If that doesn’t fix it, save the new bulb as a spare (it will be needed eventually) and then go on to testing procedures.

As GreasyJack mentioned, DON’T TOUCH THE GLASS OF THE BULB! That can shorten the bulb’s life by quite a lot.

What GaryT and others said upthread. Low beams burn out more frequently because they get used more. And parts that are replaceable are often that way because they fail more often than non-replaceable parts.

FWIW, Walmart will replace headlight bulbs for $7.50 each (assuming you bring in the parts). That is a pretty good deal and way better than screwing with your headlight assembly if you don’t know what you are doing.

Search YouTube for your make/model and ‘headlight replacement’. Changing the bulbs in modern car headlights can be really really easy or really really hard, and it all simply depends on if you can get to them (i.e. the back of the headlight under the hood). Most modern cars pack everything in so tightly they can be amazingly difficult to reach.

If you can get to them they’re pretty simple to replace: Unplug the harness and unscrew the plastic collar, so this would be the easiest & cheapest thing to try first.

I do, but I know how to handle them, because our microscope uses fancy mercury bulbs that require the same precautions.

That would be a really good deal if you had a GMC Acadia

If only one light is out, and only the low beam, it is most likely the bulb.
Fuses are cheaper than bulbs, so it can’t hurt to start there. Most fuses are designed to be visually inspected. Open the car’s fuse box, find the fuse(s) for the headlights, and see if it is blown. If you aren’t sure, then replace it. Like I said, fuses are cheap. And you should always have some spares.

There are some devices that function like fuses but aren’t in the fuse box. These are usually harder to replace, so hope it isn’t one of those.

Might be a bad connection. Wiggle the plug where the wires attach to the bulb. Unplug it and plug it back in. If that fixed the problem, yay. If it didn’t (and it probably won’t), then either it is a bad connection somewhere else (which will be hard to find and therefore bad), or it is the bulb.

If it is a cheap bulb, buy a replacement. If it is expensive, test it to see if the circuit is broken inside the bulb.

So you’ve got a brand new fuse and a brand new bulb and the light still doesn’t work? You have Bigger Problems™. Something is wrong somewhere in the wiring.
Armed with a circuit tester, a lot of patience, and hopefully a wiring diagram that tells you which color wires do what (which are a part of any repair manual for a car made before 1980, but not after for some reason), you can track that sucker down.
OR you could pay somebody to do that for you.

At this point we are into seriously low-probability events. Most likely, it was the bulb.

Fun story: In 1989 I bought a 1980 Mazda GLC. Shortly after I bought it, the headlights stopped working completely.
I took it to a shop that specialized in automotive electrical repair and asked him to answer one simple question: is there power at the switch? He said yes.
This suggested it was a “fusable link” in the wires to the headlights. Which I could not find (it would appear to be a thicker section of the wire, and I just couldn’t find anything like that).
In my growing frustration, I decided to verify the test results myself in a very primitive way: I went to the base of the steering column and found the wire that supplied power to the headlight switch, then I found the wire that supplied power to the parking light switch. I cut both, and connected the headlight switch to the parking light power.
Headlights worked fine.

Then, this being me, came the kludge. I connected both the headlight switch and the parking light switch to the power from the parking light fuse, wrapped the connection with electrical tape, and pronounced it “fixed”.
My understanding is that every time I turned the headlights on it should have blown that fuse (the headlights had previously been on their own circuit with a higher amp fuse), and I still don’t know why it didn’t, but it didn’t. The headlights worked fine for the few more months before I sold that car to a junk dealer.

But I never forgot that I had paid an electrician $30 to narrow down my search for the problem (before or after the switch), and I got the wrong answer.

It’s certainly not rocket science, but simple does not necessarily equal easy. The general steps for most modern cars are:

  1. If present, remove any rubber boot (these keep moisture out of the back of the headlight assembly). Note the orientation of the boot for reassembly.
  2. Remove the back of the headlight assembly (generally involves unscrewing a plastic ring) and slide it out of the way.
  3. Reach inside the assembly, usually where one cannot see, and determine the type of retaining system for the bulb. This usually involves a spring clip where one either squeezes the ends together or pushes the head of the clip back and to the left to release. Fortunately the clips are almost always attached to the assembly on one side and swing out of the way, so you don’t have to worry about losing the clip. If lucky it won’t involve clips but will be a twist lock set up where you twist the bulb harness until the tabs line up and slide it out. These are, to me, the easiest to remove/replace and therefore are the least common.
  4. Unplug the old bulb
  5. Plug the new bulb firmly into the wiring harness, paying attention to the warnings about about not touching the glass. I wrap the glass part in a thick, clean shop rag to make sure I don’t accidentally touch it while pushing the bulb into the plug. Thick paper towels work as well. Gloves can work, but they must be clean.
  6. At this point I turn on the headlights to make sure the bulb is working properly.
  7. Carefully insert the new bulb into the headlight assembly, making sure it seats completely (doesn’t move around when you jiggle the harness). For this step you cannot use the rag/towel/glove to protect the glass from your skin oils, so be careful to handle by the harness.
  8. Reverse steps 1-3 to reassemble
  9. Profit! (Not really, unless you do this for a living)

As suggested above, YouTube is your friend! Since about half the operation is done blind, it’s always helpful to know how everything works before starting the job. Also, it’s a given that, should you follow my instructions above step by step, your car will be different and you’ll screw something up. :smiley:

IME, the plastic ring and rubber boot are generally part of one assembly, and the bulb and its socket come out with the ring.