Hi all,

My 2007 build Focus mk2 acquired a fault today. I was greeted with the check engine light on whilst stuck in traffic.

Came home, scanned the codes and here's what came up:
P0504 - Brake Switch A/B Correlation

The problems:

If the light switch is in the OFF position and I press the brake pedal:
- brake lights go on
- dash lights, side lights, reg plate bulbs go on

If the light switch is in the ON position 1:
- dash lights, side lights, reg plate bulbs go on
- brake lights are on

If the light switch is still in the ON position 1 and I press the brake:
- dash lights, side lights, reg plate bulbs stay on
- brake lights get brighter

So now I'm thinking - ah, easy side light wire is shorting to the brake lights and that's why either turns on the other.

Here's where it gets tricky.

Engine off, key in my pocket, light switch ON in position 1 and I press the brake:
- dash lights, side lights, reg plate bulbs are on
- brake light is brighter
- ignition turns on

WTF?!

I plugged out the brake switch and the brake lights still come on when I turn on lights to position 1.

Plugged it back in and then I unplugged the light switch. Pressing the brake pedal still turns on dash lights, side lights and reg plate bulbs.

Anyone heard of this before? The ignition part really scares me though...

I had this problem when I tried LED bulbs for side lights, it was due to the resistance being too low in the circuit.

Have you recently fitted LED bulbs anywhere on the car?

If not then it may be a short or earthing fault somewhere.

Sent from my SM-G900F using Tapatalk

Have you replaced any of the brake/rear position bulbs recently? (Does the Focus mk2.5 use the dual filament bulbs?)

Edit:

I would second a grounding problem/water/moisture ingress into the passenger footwell or light clusters somewhere. Removing fuses for things then checking if the problem still exists may narrow down the location of the fault.

The filaments in one of the stop/tail lamps are probably touching, replace the stop/tail lamps.