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2005 STS
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Discussion Starter #1
My 2005 STS has a drivers side tail light out. Can I replace a bulb myself, or do I need an entire LED? Should I take it to the dealer?
 

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08 STS-4 N* 1SG, 08 DTS Luxury II, 04 Bonneville GXP N*
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3,206 Posts
These are easy to change, but they are not a serviceable item so you'll need to purchase the entire tail lamp. I'd have to check the schematic, but there is a chance left and right may be fused separately. Checking this would be a better place to start, then go the expensive route only if necessary.

I'm assuming it's the whole lamp that's out, not just some individual LED's?
 

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08 STS-4 N* 1SG, 08 DTS Luxury II, 04 Bonneville GXP N*
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The park lamps are fused separately, left and right, in the underhood fuse block. The stop lamps are fused in the right rear fuse block but both left and right are on the same circuit. If absolutely nothing works (brake, signal, park) then you have either a bad tail lamp assembly or bad wiring. My next step would be to pull the connector off the tail lamp and check voltages.

Cavity 'D' is ground and will be a black wire.
Cavity 'A' is the park lamp supply voltage and will be a purple wire.
Cavity 'E' is the brake lamp supply voltage and will be a light blue wire.

If you look very closely at the connector, you should see the cavity letters embossed in the housing. This will help you to identify the cavities.

With the park lamps on, you should have roughly 12 volts when measuring across cavities 'D' and 'A' (with the engine off). When someone steps on the brakes (car must be running), you should see about 13-14 volts when measuring across cavities 'D' and 'E'. If you have proper voltages, your tail lamp assembly is defective and needs to be replaced. If you don't, you have a wiring problem that will need to be investigated.
 

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2015 xts Premium fwd 2009 CTS AWD
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579 Posts
Do they still use a switch at the brake pedal? Could that be the problem? I don't know if he got a good reading from cavity E or not.
 

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08 STS-4 N* 1SG, 08 DTS Luxury II, 04 Bonneville GXP N*
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short in the board, youll need a new lamp
It is unlikely to be a short or it would have blown a fuse and neither side would work. If there is a problem with the tail lamp, it is most likely an open circuit due to corrosion from moisture intrusion. It's also possible there is a problem with the logic in the lamp.

Do they still use a switch at the brake pedal? Could that be the problem? I don't know if he got a good reading from cavity E or not.
Yes, there is still a brake lamp switch. Since one side works, this rules out the fuse, switch and relay (located in the right rear fuse block).

If there is any question about the supply voltage to the connector, I would double check it again. The likelyhood of wiring is small because the wire runs directly from the rear fuse block to the connector and does not pass through any other connectors. Also, none of the wiring is exposed to the exterior of the car.

I guess what I'm saying is "correct" troubleshooting technique dictates checking all wiring up to the lamp assembly prior to replacement. The tail lamp is quite likely the problem but also expensive. Rule out all other causes before spending money on costly parts.
 

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2006 STS 1SF N* RWD NAV F55
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705 Posts
Aren't we forgetting something, the brake lights signals,come through the multi-Function switch,
to cut out the brake signal, and blink the appropriate, tail light ?
I am NOT saying ,the Whole light is good or bad,
the voltage readings "NEED" to be read at the connector as:
ddalder suggests above !
Good Luck.
 

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2006 STS 1SF N* RWD NAV F55
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705 Posts
When you put your right blinker ON, do both tailights blink ?
OR only the right One,The multi-Function switch seperates
that function.
 

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2005 STS V8
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574 Posts
Both turn signals work as they should, both tail lights work as they should, and the left side and center brake lights work as they should. It is only the right side brake light that is not functioning.

Also I have noticed on the right side tail light one of the individual LED's is intermittent.. sometimes it's on sometimes it's off.
 

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2015 xts Premium fwd 2009 CTS AWD
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579 Posts
Can't you take the right taillight out and connect it to the connector for the left one? Then if it still doesn't work it has to be be the lamp assembly.
 

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2005 sts
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5 Posts
I have the same problem and I switched the drivers side with the passenger side and the brake lite still doesn't work. So I'm thinking that
there is something wrong with the pc board inside. Does anyone know how to remove the tailight lens so I can get at the pcb and check it out and does anyone have a schematic.
thanks in advance
Bob
 

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2006 STS 1SF N* RWD NAV F55
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705 Posts
Not sure what I meant, when I wrote the above, it was a year ago....
I think I meant when you put the emergency flashers ON, do they both blink...
Sorry for the confusion.
Would be interested in anyone removing the Lens to work on the PCB.
I have ONE led out on the passenger side. thanks.
Happy Motoring..
 

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Has anyone figured out any other fix for this problem other than spending the $150-200 to replace the tail light assembly?

caddieburg, Your account is now activated. Please update your profile to reflect your correct age, Cadillac driven, location and time zone (Do you really live in London ?) Use the settings, edit profile tab. Welcome to CF.
 
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