This one is a really big pickle. Tons of google results for the warning, I couldn't find anything in the 10-15 threads I looked. Most instances are just bad bulbs, which is not what this is.

Brake lights have two modes. "low" and "high". Low is for at night when the headlights are "on" so other drivers can see your car. High is whenever you press the brake pedal. The lights get brighter.

My problem is: the brake lights turn on with "high", as they do in daytime mode, but at night, they do not revert back to "low". At night, when the headlights are "on" (in any of the automatic or manual settings), the brake lights are completely shut off. When I hit the brakes they illuminate like they should, but they do not revert back to "low" mode. Turn signals function, reverse lights function. So there is power to the bulbs. It's like the rear tail lights of the car think it's in daytime mode, even though my automatic headlights function. The lights still do not work when I turn the light stalk to manual "on".

The fault "tail lamp failure" warning intermittently displays. When I start the car, it triggers once I hit the brake pedal. Sometimes it does, sometimes it doesn't. But the "low" mode on the rear brakes never illuminate. I believe it's the warning itself that's intermittent.

Every single actual bulb works. All reverse lights, all turn signals, all brake lights. The only non-functional light is the license place illumination, both lights are not turning on. However, this also would happen if the rear of the car thinks it's in "daytime" mode, even though the main headlights are triggered automatically and turn on once it gets dark. I think it's unlikely both license plate lights burned out, especially since I replaced one of them only a few years ago.

I had a rummage through the wiring harness accessible in the trunk and the two access ports for the trunk-mounted brake lights. No issues. Everything is snug and dry. Not rotted cables. No moisture. I didn't pull any panels off so I didn't investigate very far. I suspect this is a contact lead issue somewhere further up the chain which controls all the rear lights? What could be going on here.

 

Just to be clear, the rear lighting bulbs (not the BU or side markers) have two filaments in each bulb. The low wattage filament for the tail lights turn on when the headlights & parking lights are on (and the brake pedal released); the other high wattage filament comes on when you step on the brake pedal.

Sounds like you have no tail lights; no rear lights when the park/headlight switch is on. IOW, the low wattage filaments in the two filament bulbs aren't getting power. On the surface, it sounds like you have a headlight switch issue. However, more diag is necessary to pinpoint the problem.

The car doesn't "think" day/night mode. This is purely a function of the headlight switch, regardless of switch position (park/on/auto).

 

I'd check soldering cracks viewing the circuit board of the brake light failure sensor as is this carefully. I have seen boards which had cracked soldering points many in the past at local shops.

 

I have a 98' with the optional automatic self-leveling HID's, which come on automatically at night. I never turn them on or off, I just leave it on "auto". When it gets dark out or I go in a tunnel, the headlights turn on. Otherwise they're off. There is an ambient light sensor in the front dashboard.

I managed to scroll through all of my pictures and got pretty lucky. I found one from 12 months ago where my lights were functioning normally.



If I replicated this picture today (car turned on, in park, nobody in it), my headlights would be on. The fog lights would be on. the yellow side markets would be on. But there would be zero illumination from any rear light.

About the headlight switch: I can control the headlights normally, they turn on\off as expected and the automatic day\night mode works. I don't get a tail lamp failure warning during the day. It's only at night, when the rear lights should be on, does the warning (sometimes) come through. I would agree with your statement that the low wattage filament isn't getting any power. When I look at the bulbs themselves, I see the reverse lights have 2 wires, with the brake lights having 3 wires. The fuse block is a single 7.5 or 10a fuse to the rear tail lights, so if the fuse was kaput I'd imagine there would be no lights at all?

Would a failed\failing sensor board cause the low wattage circuit to fail, yet still allowing all other functions to normally operate?

Here's what my board looks like.




As I've realized this is a pretty serious problem. No rear tail lights at night is pretty illegal, not to mention dangerous. So begins another problem to track down!

 

It would be nice of you if you were able to show us the mounted side of the board.

Any way, below is for a 98-00 Celsior but I believe everything is the same with LS400. I worry that the 7.5A fuse line has some intermittent problem. The fuse 7.5A indicated by a upper red arrow is both for the license lights and 4 tail lights. Are your LS400 show all 4 low wattage tail lights not OK or just 2. If the license lights are OK and only low wattage tail lights are not, it means that the lower red arrow part may be failed. If so, depending on 4 or 2, you can find the next step to check which is the pin 16 line in the sensor unit.

The unit is sensing the low wattage tail lights using 2 separate current sensing resistors that are connected to the pin 16.

 

Oh my. I should have looked at the fuse diagram more closely beforehand. There are two seperate fuses for the tail lights!

According to my owners manual, (page 252), the fuse block in the driver footwell contains the following two fuses

Fuse #15 Stop lights, high mounted stop light. These, I imagine, are the high-wattage brake light circuit.
Fuse #19 Tail lights, side marker lights, license plate lights. These, I imagine, are the low-wattage tail light circuit.



Something I didn't mention because I couldn't imagine it was related is - I got into a incident where someone pulled out infront of me. I had two choices: hit them, or jump a curb. I chose the latter. Thankfully, there is no mechanical damage (it didn't even throw the alignment out!) However, it pulled the front bumper slightly, cracking it in two small places, and breaking the bumper cover clip in the left wheel well. The other 17 of whatever clips remain attached so in the scheme of things, I am very fortunate.

However. As part of this incident, *drum roll*, my passenger side side marker light is no longer working! Which is on the same circuit as the license plate lights and the tail lights! This is a very strong correlation and I suspect where the problem lies. Probably a short, or the cables got stripped from the light getting "yanked" out of the plug. I'll have to take a closer look at it.

The troubling part, to me, is that this incident happened about a month ago. I didn't get the "tail lamp failure" warning until 3 days ago. So that means I was driving with no tail lamps since then. Yikes. This also does not explain the "tail lamp failure" warning I got last year. There is a problem with Fuse #19, and, there may also be an independent problem with the "tail lamp failure" system that existed before my accident.

I will investigate the side marker more closely tomorrow. I was originally going to solve the problem in a few weeks once I can gain access to a lift, since it's entirely cosmetic damage. However, in light of this being a safety-critical item for driving at night, I will be addressing this on Monday.

Thank you gentlemen for your input & you guys reminding me to look at the layout for what is actually connected to what circuit. I am not sure I would have arrived here without your help.

 

I just added names of bulbs in English using red letters for your convenience.
I also need to add that wires 白ー黒 means White-Black. All those ground lines for small wattage bulbs are White-Black. I worry that some of those White-Black wires are damaged or grounding points of them are not fully conductive caused by the accident.

 

I already buttoned up the sensor again. When I had it out, I took a look at the front side of the board and didn't notice anything amiss there either. No burn marks, damage or evidence of the "magic smoke". If other investigations are unsuccessful - I will take it apart again and take more picture(s) and have a very very close look at the locations you've labeled. Thank you for the translations and the diagram.

It's possible the wires got pulled out from the light when the bumper got stretched, which pulled out and exposed wire, which could potentially be shorting on something. Causing the grounding issue like you say. I would hope a situation like that would have tripped the fuse which would protect the rest of the system. I will investigate further.

 

Finally got the car up. Unbuttoned the aero covers under the front bumper which you can move aside and grab the bulb from. People on google were removing headlights or fender liners to access this thing, idk what they were thinking. It's like 7 screws and you can easily pull the plastic back and grab it.

A ton of water came out of the hose that the wire runs up, but considering it's open to the elements I'm going to imagine that's normal. Some snow or whatever probably got jammed up there over the winter and can't evaporate fast enough.

I would love to pull the whole connector apart and check it, but I can't for the life of me un-clip the plastic holding the pieces together.

I don't see any connection or arcing or burn marks or corrosion issues. Bulb looks fine. Connector behind the bulb looks fine. Cable & plugs leading into the harness looks fine.



Checked the fuse box next.

Well there's your problem.



ding ding ding. Problem, at least for now, solved.



This is with the front side marker bulb removed. Will test again later with the bulb put back in.