06 CRD P0504 fault code help

Steve777 · **Joined:** Fri Oct 16, 2015 11:31 pm **Posts:** 284

Been dealing with a P0504 code, stop light switch redundant circuit mismatch, for a while now without much luck. I am not exactly new to this code, have had it pop up now and again on other CRDs, but it was never this this hard to track down and solve. I have tried several things only to have the code come back, usually fairly quickly (like within 10 mi of driving). I have read many posts on this problem but have not yet come up with the solution. Here is what I have tested/tried/done so far, in case something jumps out at someone:

1. All the brake light work fine, did from the beginning.
2. Checked fuse #12, it is fine.
3. Replaced the stop light fuse (aftermarket).

None of these things pointed to any problem, and the new stop switch did not change anything.

When I look at the code on my reader, it typically records the stats of when the error happened, and it usually is at 0 MPH, so stopped (likely at a light). Not sure what if anything this may mean.

Have looked in the 06 Service Manual, pg 9-990 for the diagnostic steps for this code, and unfortunately they assume you have a dealer code reader to check a bunch of stuff, so not able to get too far with this. When the weather improves (around 0F now) I could check the simple voltages and for shorts there, but given all the stop lights are working and bright I'd expect those to be in good shape.

Thought I should unplug and check the main computers connector for that pin, but it isn't loose so unlikely to be the problem, but suppose it could have some corrosion.

Bad replacement switch? Maybe, but I have used this brand before and they were fine.

Looking for ideas of what to check next. Does the error happening at 0mph give any clues?

WWDiesel · **Posted:** Sun Nov 30, 2025 2:02 pm

P0504 -- Brake switch – A/B correlation Wiring, mechanical fault

Found this, maybe it can be of help in finding your problem? The item below (**) in bold red text is most likely key to the problem.
HINT: I would also check all voltages, fuses, signals, grounds, etc... at their respective locations with a voltmeter, while operating the brake pedal to ensure they are providing the ECM with the correct signal for their respective positions.

Quote:

The P0504 code means there's a correlation fault between the two signals from the vehicle's brake switch, indicating a problem with the switch, its wiring, or the brake pedal adjustment. This can cause symptoms like a check engine light, non-functional cruise control, and incorrect brake light operation.
**The underlying issue is that the vehicle's computer (PCM) receives contradictory signals that the brake pedal is both pressed and released at the same time.

Causes of a P0504 code
Faulty brake light switch: The switch itself may be damaged or malfunctioning, leading to incorrect signal output.
Brake pedal adjustment: If the brake pedal is out of adjustment, it can prevent the switch from functioning correctly.
Wiring issues: A short circuit, open circuit, or damaged wiring harness connected to the brake switch can cause an improper signal.
Blown fuse: A blown brake light fuse can interrupt the circuit and cause this code.
Mechanical obstruction: Debris or dirt can physically prevent the brake switch from operating properly.
You can watch this video to learn how to diagnose and fix P0504 code:

Quote:

How to diagnose and fix

06 CRD P0504 fault code help

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