My '99 LS400 has a Taillamp Failure message coming on as soon as I touch the brake pedal. The outer bottom bulb (only bulb on outer end of the harness) is out on both sides. I have replaced both bulbs but still doesn't light up or change message. Likely issue? Find it weird that both are out and if I didn't know it was out I would never have noticed. All actions seem to be working (brake, turn, backup) so not even sure what it is for.

Is your 3rd brake light working? (Back window) Sometimes those bulbs go out and ya don't notice it.

i agree with the 3rd brake light. check that first then make sure the parking lights (dim) are also working!

normal (parking lights):

I don't think it's a 3rd brake light issue because your words,"both bulbs but still don't light up or change message" is telling so. It must be related to the tail light failure sensor located in the trunk as Timmy kindly posted. I assume there is a soldering crack similar to this or the board is damaged thermally. The link is about a 90 LS400 and the printed circuit board is a bit different but it's quite similar to a 99, I must add.

I had replaced the back window brake light bulbs first and both are working. I'm assume that there is an independent circuit board for each side? Don't know why bulbs one each side would be off simultaneously though.

Because the sensor circuit is supplying the current to 2 bulbs of left and right simultaneously. When the sensing resistor's current path is open, there goes no current and new bulbs won't be driven, Why don't you simply check the board? 　

Will do. Thanks.

OK...problem resolved. After checking the circuit board and not finding any issues. I found a thread that talked about double checking the 3rd brake light bulbs even if they were working. Sure enough the replacement bulbs that I had installed which were recommended by my local auto parts store worked but were not the correct bulb listed for my Lexus. I bought correct bulbs at W Mart and now warning message is gone. Thanks for your help everyone!

Glad ya figured out it was the 3rd brake light. I had problems with my 94 3rd brake light. It kept setting the light failure message (only when it was cold outside) even though all the bulbs were good. I would pull the bulbs out and put them right back in and it was fine for a while. I never figured it out so I got no answer there. I just dealt with it and checked all my lights once a week anyway. No big deal cuz I did that on the trucks I used to drive so it was like second nature.

You should always do your own research online to get correct part numbers before going into one of the chain store part stores. Most of the kids working there don't really care if you get what you need, they just want ya outta there so they can get back to playing their games.

Can you share what size and watt bulb you ended up using? I'm having the same issue. Looked up the bulb size and it says it should be a 921 so I bought some Sylvania 921 long life bulbs and they work but i still get the 'tail lamp failure' message. The only difference I can see between it and the original OEM bulb is the Sylvania is 17.9 watts and the OEM is 16 watts. Not sure if that would make a difference or not.

Check the fuses (in the driver footwell) and the licence plate illumination light.

The amber side markers on the front bumper are also on the same circuit as the license plate light and the tail lamps (tail lamps are different than brake lights, the latter of which come on when you hit the brakes).

higher wattage should be fine. it's the lower wattage that causes the taillamp failure to pop up (such as running LED lights).

if you're still getting the failure, check to see if any of the running (parking) lights are also functional. it is not just the brake lights the system finds that fail.

Thanks...finally figured it out. The 2nd filament on one of the dual filament brake light bulbs was out.