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I think the Bulb Failure Sensor is OK, and doing "its thing" by alerting you to the two failing bulbs. Rob's tips on cleaning up at the sockets will probably fix you up.
FWIW, here is my canned BFS description, followed by how the 3 Brake Light failure detection works (for the trivia lover in us all)...
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The Bulb Failure Sensor tests for a current balance in each of these three groups of light pairs (includes 3 bulb brake lights):
• Tail lights
• Brake lights
• Low headlight beams
You can isolate the BFS warning light to one of these groups as described below. The car must be running to activate the Bulb Failure sensor:
1 – Step on Brakes.... If no Warning light,
2 – Turn Parking (tail) lights on.... If still no Warning light,
3 – Turn on Low beams. Should be the cause of the Warning light.
If Warning light is ON when all these lights are OFF (or all tested lights are OK), it could be the BF Sensor at fault.
However, "mismatched" bulbs (different brands, age, or socket condition), or corrosion at the common ground points for Left & Right light housings ) in any of the 3 light groups can also cause a false BF light.
Note that there will be NO warning if BOTH bulbs in a pair are out.
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How the system checks the three Brake Light bulbs
This bit of Volvo trivia had me stumped until I autopsied a BF Sensor. The basic principle is that.an equal current thru two opposing coils (having the same number of turns) will have no magnetic effect on the reed relay inside the coils.
The 3-Light Brake Light test circuit is similar, except that the Left bulb is tested by one coil (say 20 turns), opposed by two coils end-to-end, 10 turns for the Right bulb and 10 turns for the High Stop bulb.
As long as all three bulbs work, the 20/20 coils cancel out. However, a failure of any one of the three bulbs will upset the 20/20 "magnetic balance" and the sensor relay will light the BF Light.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.
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