OBD-II Code P0504 — What It Means & How to Fix It

What Does This Error Mean?

P0504 means the two brake switch signals — switch A and switch B — are not agreeing with each other. Modern vehicles use two brake switches on the brake pedal for redundancy and safety monitoring. When you press the brake pedal, both switches should activate at essentially the same time. If one switch signals 'brake pressed' while the other says 'brake released', P0504 is stored. This can affect brake lights, cruise control, ABS, and traction control.

Affected Models

Most 2000 and newer vehicles with dual brake switch monitoring
Very common on Toyota and Lexus vehicles
Common on Honda, Nissan, Ford, and GM vehicles
Any vehicle where the PCM monitors both a normally-open and normally-closed brake switch simultaneously
Vehicles with advanced driver assistance systems that depend on brake pedal status

Common Causes

One brake light switch has failed mechanically — contacts worn out or stuck in the wrong position
One switch is misadjusted — it activates too early or too late compared to the other switch
Brake switch wiring connector is loose or corroded on one of the two switches
A brake switch is binding on its mounting bracket or the pedal arm, causing delayed activation
Stop light fuse or circuit fault affecting one switch's circuit but not the other

How to Fix It

With the ignition on, press the brake pedal and watch the brake lights. Both brake lights should illuminate immediately. Then check whether the brake light status on a scan tool shows 'pressed' in sync with the physical pedal. Mismatched status is a clear sign of switch correlation failure.

Some scan tools show switch A and switch B status separately as live data parameters — this is the most direct way to see which switch is behaving incorrectly.
Locate the brake switches above the brake pedal arm. There are typically two switches mounted on a shared bracket near the top of the pedal. Check each switch to see if it moves freely and activates with the correct pedal travel.

One switch is usually normally open (activates when pedal is pressed) and one is normally closed (opens when pedal is pressed) — their signals should be opposite each other.
Inspect both switch connectors for loose connections, bent pins, or corrosion. Disconnect and reconnect each one. Clear the code and drive the vehicle to see if the correlation error returns.

A connector that vibrates loose on the brake switch is a very common cause of intermittent P0504.
Check brake switch adjustment. The switches are typically held in place by a nut or a twist-lock mechanism. If one switch has shifted position slightly, it may not activate at the correct pedal position relative to the other switch.

Most brake switches are adjusted by pushing the switch forward until it contacts the pedal arm stop, then locking it in place. Consult your service manual for the exact procedure.
If adjustment and connections check out, replace the failed switch. Brake switches are inexpensive and widely available. Replace only the switch that the scan tool identifies as behaving incorrectly.

After replacing a brake switch, verify brake lights work correctly and that cruise control engages and disengages normally — these are the most immediate functional tests.

When to Call a Professional

Brake switch replacement and adjustment are straightforward DIY tasks. The switches are usually visible and accessible above the brake pedal without any major disassembly. Costs are typically $15 to $50 for the part. If the issue involves ABS or VDIM modules, professional diagnosis may be needed.

Frequently Asked Questions

Will P0504 disable my brake lights?

Not usually — a single working switch is typically enough to keep the brake lights functioning. But if the failing switch is the one controlling the brake lights, they may malfunction. Always verify your brake lights are working normally whenever you see a brake switch code.

Can P0504 disable cruise control?

Yes. Cruise control relies on brake switch input to disengage when you press the brake pedal. If the PCM cannot trust the brake switch signals, it will disable cruise control as a safety measure.

How many brake switches does my car have?

Most modern vehicles have two brake switches mounted near the top of the brake pedal. Older vehicles (pre-2000) often have just one. P0504 specifically indicates a mismatch between the signals from both switches — so two switches are always involved in this code.