One headlight shuts off and eventually goes back on

Light's going bad, replace it.

 

HIDs do not fail like filament bulbs. They 'blink' in failure.

 

If you have the Xenon HID headlights, this is symptomatic of your HID lamps going bad. It is a chronic problem with the Gen II Prius that Toyota will not fix for free. They have reimbursed some owners for the cost dealers charged them to replace an ECM that wasn't the issue., but not for the bad batch of lamps.
I suffered with mine for months before replacing. A few minutes after starting the lamp would blink off. I discovered if I recycled the headlight switch it would come back on for a while. Don't suffer, just replace both lamps.
Dealers say they have to remove the bumper cover to do it, about $200-$300. It's not so. You can do this yourself if you have some manual dexterity and hand strength. Luscious Garage website has a video of this and blogs of many useful Prius tips. I've attached instructions. I recommend Sylvania/Osram or Phillips D4R lamps. Prices are going down through Amazon.

 

If it's the 3rd bulb this year, you may have an electrical problem.
One question first. Do you turn your lights off and back on in less than 2 minutes frequently, like going through a security checkpoint? This is a HID lamp killer.

 

I think I may have the same problem. The drivers side low beam goes out when it turns cold or temperature changes. My mechanic replaced something last year - the sling?- that we thought would fix it but it didn't becuase it happened again this year. Today though, both low beams have gone out.
If I'm understanding this thread correctly, I should replace both bulbs? I don't have HID (no fog lights)

 

I'm an optimist, so I always check the easiest cheapest solution first.
If there was work done last year and it's a sporadic issue, there may just be a loose connection.
If you indeed have the halogen, when they burn out they are just gone, no dying to it.
Check fuses 26 (Right lo-beam) & 27 (Left lo-beam) in the engine compartment first. Both 15-amps.
Make sure they're good and fully inserted.
If they're good, check the connections on the lamps, but be prepared to replace the lamps when you get the socket covers off. Halogen don't necessarily turn black and can't always see the broken filament.
The good news is that if you have halogen instead of HID, they're much cheaper. Lamp #9003.
Check out this link first to figure out what you have.
The socket cover for HID has tabs sticking out to help remove it.

Toyota Prius: How to Replace the Headlight Bulbs (HID and Halogen) - PriusDIY.com

With a couple basic tools like a Phillips screwdriver to remove the radiator support cover you can replace the lamps.

 

Well this explains what I am experiencing with my '04. Here in Ohio, we get lots of gray sky days and drizzle, so the headlights are almost always on. One of mine has been "lazy" lately. Now I know why.

 

I can't get in to the mechanic until Monday because of the holiday's but I will let him know about the suggestions. I tried looking and poking around but it's just beyond me. I can handle anything before 2000, anything newer is just overwhelming.

 

It is a little difficult doing the lamps if you're not a do-it-yourselfer. The fuse box is pretty easy. Just pull out on the back of the cover on both sides and it will come off. There is a white fuse puller on right side below the 4 fuses located there. There is a diagram of the fuses and brief description on the inside of the fuse box cover. You can just pull 26 and 27 and see if the filament is burnt. There is usually a spare 15 amp in the center row near the top. 15-amp are light blue.

 

will let you know

 

updates: both bulbs were replaced, they were burnt. Unfortunately the only way to know if that solved the "going out" problem with the drivers side is to wait and see. Since it happens when the weather changes, I'll have to see over the next month if it was the bulb or something else

 

Let us know how it goes.
One thing I didn't ask before. Something that really reduces the life of the HID lamps is turning them off and then back on again in less than about two minutes.
If you drive with them on day or night and make a lot of quick stops shopping or work somewhere where you have to turn the headlamps off as you go through a security checkpoint and then back on this will drastically reduce the life of the lamps.
If this is true in your case, try avoiding doing this somehow.

 

thank you, I don't need to do that, I always have them on, the setting that is for that. The left (passenger side) bulb was the factory bulb so that lasted about 7 years. I'm not sure that the drivers side had been changed or not but I'm still suspicious because this happens each time I change climate or the weather changes. Thinking about it, it seems to happen only in the fall. So we will see next year what happens but if there is anyone else out there who has this problem and has figured it out, please let me know.

 

Hey Prius club, I'm back and "it's" back.............. ok, last time we chatted, both bulbs for low beams had been replaced. No problems, didn't think to regularly check until the other day someone commented that the passenger bulb was out. major groan on my part.............. called the mechanic, took it yesterday, he replaced the bulb and I'm on my way. Today I go through the car wash and then am vacuuming when I walk around the front and ..... GRRRRR....... not only is the light out, it has exploded. There are glass pieces in the headlamp housing. I drove straight to my mechanic to show him. He's been my mechanic for 14 years (just an FYI) and we are both stumped. So I'm thinking back over the past month for any clues and it occurs to me that I saw the master warning light display itself whenever I start the car in the morning. It may be whenever I start the car, but it lights up along with all the other dashboard lights that I am used to seeing. Other than that, I absolutely cannot think of anything else odd. It got cold here low about -1 (which is cold for Boise) and one morning the car complained about starting, displayed that light so I turned it off, put the fob in the slot and it started. But that's it, I can't think of anything else.
Thoughts?! And I just lost my job so I am hoping this is a simple fix.

 

So NOW you tell me the battery test kills my bulbs!

I've been running it several times a day to see if charging ect had an impact.

 

No need to flash the headlights, just flash the parking lights when doing the test. Try it, works for me.