On my Ford C-max, the lower 4 red rear lights don’t get brighter when braking, but the center light in the rear window does. All other lights are working. I pulled off one of the brake light housings and verified the bulb’s filaments were intact. Is there some part of the circuit that only goes to the lower lights? Other ideas?
Could be a fuse, could be a connection, or it could be something in how the lights are controlled. Lots of stuff goes through electronic controls now. My 2011 MKZ had a recall to fix how the computer controlled the taillights. What year is your C-max? It uses bulbs, not LEDs?
2013 Ford C-max. It uses bulbs.
All the tail lights are controlled thru the Body Control Module.
If all the other tail lights work except the brake lights, it points to a problem with the BCM.
Tester
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how did you verify filaments were intact?
If this is a dual filament bulb, it can be sometimes hard to tell from just eyeballing it to see if they are working.
I have no history on a 2013 Ford C Max, but my noggin tells me brake light bulbs fail more frequently than BCM’s do.