BrakesGeneric OBD-II codeUrgency: Garage soon
Code P0504: brake switch A and B signals do not match
P0504 means: brake switch "A"/"B" correlation.[1][2][3] The ECU has detected that the signals from brake switches "A" and "B" do not agree with each other as they should. Autodoc Club describes it as a correlation between the two brake light switches outside the expected parameters, and CarParts.com as an incorrect signal or an electrical fault in the brake switch, which normally closes when the pedal is pressed.[1][4]
What code P0504 means
The ECU has detected that the signals from brake switches "A" and "B" do not agree with each other as they should. Autodoc Club describes it as a correlation between the two brake light switches outside the expected parameters, and CarParts.com as an incorrect signal or an electrical fault in the brake switch, which normally closes when the pedal is pressed.[1][4]
That switch does more than turn on the brake lights. Autodoc Club points out that the fault also affects cruise control, and Rick's Free Auto Repair explains that modern brake switches use potentiometers that send a variable signal when the pedal is pressed and released, and that this signal is also used by automatic stop-start systems and by the solenoid that locks the gear lever.[1][2]
It is a generic OBD-II code (it starts with P0), so it means the same on any make. AutoDTCs classes it as moderate severity: it should be fixed soon, although it does not normally leave you stranded.[3][4][5]
Original definition (SAE J2012): Brake Switch "A"/"B" Correlation[1][2][3]
Causes of P0504
From most to least likely, according to the sources.
- 1Faulty brake light switch. Rick's Free Auto Repair states that in most cases it is the switch that fails.[1][2][4][6]
- 2Switch incorrectly adjusted, or a damaged or misaligned brake pedal.[1][2][4]
- 3Damaged switch wiring, or corroded or loose connectors.[1][4]
- 4Blown brake light bulbs, or a faulty brake light circuit fuse.[1][6]
- 5Electrical supply problems in the brake circuit.[1]
- 6ECU (PCM/ECU) fault or a fault in its programming.[1][4]
Symptoms
- Brake lights that do not come on when the pedal is pressed or that stay on with your foot off the pedal.[1][2][4][6]
- Cruise control that does not work or that switches off unexpectedly.[1][2][4]
- Gear-change problems in automatic cars, or being unable to move the lever out of P after starting.[1][2][4]
- The engine stalls when braking.[2][4]Engine stalls: faults that cause it
- Difficulty starting if the car does not detect that the brake pedal is pressed.[1]
- Additional ABS warning lights in the instrument cluster.[1]ABS/ESP light: faults that cause it
How to diagnose P0504
In order: from the most likely and cheapest to check to the most expensive.
- 1Check whether the brake lights come on when you press the pedal and go off when you release it. According to CarWiki.de, if both lights stay on all the time, the switch is definitely faulty.[2][6]
- 2Check the brake light bulbs and the fuse for their circuit, and check the power supply.[1][6]
- 3Inspect the switch wiring and connectors: look for damage and clean off any corrosion.[1]
- 4Check the position of the brake pedal and the switch adjustment (the distance at which it operates).[1][2]
- 5Test the switch with a multimeter, comparing against the values in your car's workshop manual.[2][4]
- 6If the switch fails, replace it and adjust it according to the manufacturer's instructions. Then clear the code and do a road test.[1][2]
- 7If the procedure is unfamiliar to you, leave it to a garage: CarParts.com recommends consulting the car's manual and going to a professional if you are not used to this type of repair.[4]
Can I keep driving with P0504?
Garage soon
The sources agree on the essentials, with nuances. Autodoc Club says you can keep driving but that it is not advisable, because brake lights that do not work are a significant safety risk. AutoDTCs classes it as moderate: fix it soon, normally without being left stranded. CarParts.com recommends avoiding driving if the brake light switch is confirmed to be faulty, until it is replaced. The sensible course is to check the brake lights straight away and repair the switch as soon as possible.[1][3][4]
Documented faults related to P0504
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
Brake pedal, hoses and brake fluid: the most serious faults by engine and car
- Critical severityFrequency: Very frequentOpel Corsa FBrake fluid leak and loss of ABS/ESP from the EBS valve block (2024 production)The manufacturer recalled Opel Corsa F cars built between 24 May and 27 August 2024 because of an EBS valve block with a casting defect that can cause an internal brake fluid leak and the loss of stability control, ABS and electronic brake distribution.
- Critical severityFrequency: Very frequentPeugeot 208 IIBrake fluid leak and loss of ABS/ESP due to the EBS valve block (2024 production)The manufacturer recalled Peugeot 208 IIs built between 27 May and 6 June 2024 over an EBS valve block with a casting defect that can cause an internal brake fluid leak and loss of ABS and ESP. The Opel Corsa has an equivalent campaign with different dates.
- Critical severityFrequency: CommonMercedes-Benz Clase E W211SBC (Sensotronic Brake Control) electrohydraulic braking system failuresThe E-Class W211 introduced the SBC electrohydraulic braking system, which could fail because of a faulty control of its high-pressure pump, especially in extreme situations; Mercedes-Benz recalled it twice (2004 and 2005) and ended up removing it completely in the 2006-2007 facelift after customer complaints.Appears at around 120,000 km
- Critical severityFrequency: CommonOpel Astra KBrake master cylinder contaminated by clutch debrisYes: Opel recalled 2016-2018 Astra K cars because the seal on the clutch slave cylinder can release particles that contaminate the brake master cylinder, lengthen the pedal travel and reduce braking power.
Headlights and lights: the most serious faults by engine and car
- High severityFrequency: CommonPeugeot 307Exterior light failure without warning (2006-2009 production)On cars built between 2006 and 2009, the exterior lights can go out or fail without warning; Peugeot issued two recalls for this reason, and gebrauchtwagenberater.de independently documents a lighting problem on models from those years.
- Critical severityFrequency: OccasionalAlfa Romeo TonaleFaulty wiring under the floor: loss of lights, power steering and brake servo (Tonale 2022-2023)Yes: a Safety Gate alert from March 2023 (campaign 6574) covers 607 Tonales built between 15 September 2022 and 13 February 2023, whose wiring under the floor can knock out the lights, hazard lights, power steering and brake servo.
- High severityFrequency: OccasionalCitroën C-CrosserOut-of-specification electronic component in the built-in systems interface (BSI): lights, wipers and sat nav may failYes: an electronic component in the Citroën C-Crosser's built-in systems interface (BSI) may not meet the specification and cause the audio-navigation, the wipers and the exterior lighting to fail, with a warning light on the dashboard. The DVSA states that the BSI's build date is checked and the unit is replaced if it falls in the affected range.
- High severityFrequency: OccasionalCitroën C-CrosserBrake light switch with contacts contaminated by silicone greaseYes: in 2014 a campaign was registered because the brake light switch on the C-Crosser, built between April 2007 and June 2009, could have its contacts contaminated with silicone grease from the factory, and the brake lights might not come on when the pedal is pressed. The switch is replaced and its wiring harness cleaned.
Wiring, connectors and short circuits: the most serious faults by engine and car
- Critical severityFrequency: OccasionalBMW N62 / N63Additional turbo coolant pump with a risk of short circuit and fireIn 2011 BMW recalled 5, 6 and 7 Series, X5 and X6 models built between 2007 and 2011 because of the additional turbo coolant pump: moisture, a short circuit and, in extreme cases, a fire. In 2012 there was another campaign for a crack in the pump, with a more robust version.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Main relay of the power relay assembly (overheating and fire risk)Yes, on the first units: Hyundai and Kia recalled 2016–2018 Ioniq and Niro hybrids and plug-in hybrids because the main relay that connects the high-voltage battery could come loose, overheat and, in the worst case, cause a fire.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Clutch hydraulic actuator (HCA): leak or moisture with risk of short circuit and fireYes, this is the most documented hybrid system fault: the hydraulic actuator that engages the combustion engine with the hybrid assembly has had two recalls (2018 for a leak on the Niro and 2025 for moisture in its electronic board on the Niro, Ceed and XCeed PHEV), with a risk of short circuit and fire.
- High severityFrequency: Occasional1.3 TCeBlown fuel pump fuseOn the Renault Captur II and Clio V with the 1.3 TCe engine, a 20-amp fuse can blow and leave the car without an engine. According to L'argus, in April 2021 this fault still had no factory fix; the official service centre's diagnosis includes checking, and sometimes replacing, the catalytic converter.
- Critical severityFrequency: CommonRover 75Seat SRS connector fault with airbag and seatbelt warning light onAuto Express warns that the Rover 75's seat SRS connector can cause the airbags to fail, and dealers were fixing it with revised connectors. The Dutch APK inspection puts the airbag or seatbelt fault warning among the most frequent defects across almost all series.
- Critical severityFrequency: OccasionalAlfa Romeo GiuliettaShort circuit from badly routed wiring that can prevent gear shifting (KBA campaign, 2010-2015)Yes: according to Germany's KBA, on Alfa Romeo Giulietta cars built between 2010 and 2015 inadequate cable routing could cause a short circuit and, as a result, gear-shift failure.
Frequently asked questions
Can I drive with code P0504?
You can, but it is not advisable. Autodoc Club warns of the safety risk if the brake lights do not work, AutoDTCs recommends repairing it soon, and CarParts.com advises against driving with a faulty brake light switch until it is replaced. Before setting off, check that the brake lights come on and go off normally.[1][3][4]
What does "A/B correlation" mean in P0504?
That the ECU has compared the signals from brake switches "A" and "B" and they do not agree as they should. Rick's Free Auto Repair explains that modern brake switches use potentiometers that send a variable signal when the pedal is pressed and released.[1][2]
Why does P0504 affect cruise control and the automatic gearbox?
Because the brake switch signal does more than turn on the lights: cruise control also uses it and, according to Rick's Free Auto Repair, so do the solenoid that locks the gear lever and the automatic stop-start systems. That is why the sources list failing cruise control and difficulty moving the lever out of P among the symptoms.[1][2][4]
Can I repair P0504 myself?
Checking the brake light bulbs and fuse is simple. Testing the switch requires a multimeter and the values from the workshop manual, and replacing it requires adjusting it properly on the pedal. CarParts.com recommends going to a professional if you are not familiar with this type of repair.[1][2][4]
Does P0504 mean the same on all makes?
Yes. Codes that start with P0 are generic OBD-II codes and have the same definition across all manufacturers, although the repair procedure changes by model.[4][5]
Will I pass the MOT with code P0504?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[7]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- Código de avería P0504Autodoc Club · Technical documentation · consulted 4 October 2026 · in Spanish
- Generic OBDII P Codes P0500-P0599Rick's Free Auto Repair Advice · Other source · consulted 4 October 2026
- P0504 - Brake Switch A/B CorrelationAutoDTCs · Technical documentation · consulted 4 October 2026
- P0504 Code: Brake Switch A/B CorrelationCarParts.com · Technical documentation · consulted 4 October 2026
- On-board diagnosticsWikipedia · Other source · consulted 4 October 2026
- OBD Fehlercode P0504CarWiki.de · Technical documentation · consulted 4 October 2026 · in German
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish