2012 Fiesta 1.25

When I have the headlights on, the brake lights are constantly on too.  Also, when I have the headlights off, the brake lights work fine but the front passenger side head lights also comes on when the brake pedal is pressed.

Running a quick diagnostic on it shows P0504(00) error code - Brake pedal position switch A/B - correlation.  Status: pending, confirmed. Test failed since last DTC clear.

Anyone had a similar experience to this and tips on a fix?

 

There is a short somewhere.  I suspect it is around the tail lights somewhere, as brake lights and headlights wouldn't generally be connected.  The alternative is a fault at the BCM where all of the light control passes through.  First check would be tail light boards and plugs, then BCM plugs.

2 hours ago, Felix. said:

Anyone had a similar experience to this and tips on a fix?

Very common fault, and even easier to fix.

Replace BOTH the Stop & Tail light bulbs. All that has happened is that there is a short circuit inside one of these bulbs. From experience it's quicker and easier just to replace both bulbs.

Have you changed any bulbs recently?
Possibly the wrong bulb type fitted somewhere that is backfeeding the lighting circuit.
Worth checking all the bulbs for correct type and fitment

Funnily enough, it went for an MOT test last week and they changed the o/s/f indicator bulb among a few others apparently.

I might change them back to see if it makes a difference.

So, it turns out that the n/s/r bulb was a single pole one instead of a double.

I can only assume that they have put the wrong bulb in - which begs the question as to how a garage mechanic can get that wrong?????

Anyway - all working again now