All,
Bought a 95 940 two weeks ago. While driving home the bulb out warning on the dash came on - the left brake light burned out. So when I arrived at home I purchased 12 bulbs and replaced all bulbs in the back at once (figuring most were original). Today the warning light came on again and now the right brake light is out (the left brake light and the window ledge lights are working). I inspected the bulb - it looked good, but to be safe I put in another new one. Still out. Swapped out the 3-bulb appliance holder from my other 940 thinking this was bad. No change. Next I inspected the wires in back - don't see any fraying or cracks. I'm guessing the brake pedal switch, bulb relay, and fuse are all OK - if faulty all three brake lights would be inoperative. One last item - the records from the previous owner indicated the bulb relay was replaced last fall because all brake lights became inoperative - further evidence that the relay is probably OK since very new. So what do I look for next? Appreciate any advice.

Likely a bad ground connection or corrosion from the bulb holder to the ground plane nearby.

we have two 95 940s, a wagon and sedan. Sedan gets intermittent bulb failure warning light. I have replaced bulbs, etc... then thought it was the severly crimped wire harness near the lamp connector (the wire harness is crushed when the plastic protective cover is screwed onto the back of the lamp fixture), but the wires were ok. I have always had a good ground. I am now fairly sure the problem is due to the long-term loss of tension on the spring steel contactors that the bulb rests against on the bottom of the bulb receptacle. Adding to the problem is the fact that the bottom of the bulb is made with ultra SOFT lead that becomes deformed due to the pressure of the spring steel contactor. The light clears when I put in a new bulb, but after about 3 weeks, it is back again, until I hit another bump and it goes off. But, it will return sooner or later.

I realize it could also be the relay, but it tests OK.

"...bulb relay, and fuse are all OK - if faulty all three brake lights would be inoperative."

Bulb Failure Sensor could still be at fault. It will only kill all 3 brake lights if the single common input fails, But there are 3 separate outputs—one for each light. Lots of "new" parts come from less than reliable sources these days.

The output for the right brake light is the Yellow wire from terminal 11 (54R).

I'm sorry that this info probably isn't much help on a 900, with its inaccessible relay panel. It would be a breeze to check on a 240. Try another BFS if possible.
--
Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

Two more things...
Check to see if you have power to the bulb socket when the brake is applied.

Second, check to see if you have a good ground by using a continuity meter from the socket to a body ground like the trunk latch.

After that we can look into other things..