P0504 fault code: Brake Switch 'A'/'B' Correlation

Last updated · OBDCode UK editorial

Can I drive with P0504? — Drive only if essential

Limit driving to essential journeys. Avoid long trips and heavy loads. Book diagnosis soon — if the car overheats, loses power, or feels unsafe, stop and arrange recovery.

P0504 means “Brake Switch 'A'/'B' Correlation” — a vehicle speed and idle control fault in the powertrain.

Can I DIY?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.

Caution — conditions applyDIY boundary
ToolsBasic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.

P05xxVehicle speed and idle control
MOT fail riskif the warning light stays on

What does P0504 mean?

P0504 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P05xx block, which covers vehicle speed and idle control. The stored description is “Brake Switch 'A'/'B' Correlation”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0504 fail an MOT?

An illuminated engine management light is a Major defect at MOT since 2018 — an automatic fail until the fault is repaired and the light stays off. Clearing the code without fixing the cause is not a repair.

Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.

What the MOT data says

No MOT certificate item corresponds to this code; MOT outcome data cannot be shown.

Related guides

Car diagnostic test cost: compare the scopeUK booking marketplaces currently publish diagnostic averages of about £52–£65. Engine management light on: steady or flashing?A steady engine management light means the computer has stored a fault. Drive with caution and get the code read. A flashing light is different — stop when it is safe.
P0500Vehicle Speed Sensor 'A' P0501Vehicle Speed Sensor 'A' Range/Performance P0502Vehicle Speed Sensor 'A' Circuit Low Input P0503Vehicle Speed Sensor 'A' Intermittent/Erratic/High P0505Idle Air Control System P0506Idle Air Control System RPM Lower Than Expected P0507Idle Air Control System RPM Higher Than Expected P0508Idle Air Control System Circuit Low

P0504 — quick answers

Is it safe to drive with P0504?

Drive only if essential. Limit driving to essential journeys. Avoid long trips and heavy loads. Book diagnosis soon — if the car overheats, loses power, or feels unsafe, stop and arrange recovery.

Can I fix P0504 myself?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Will P0504 fail my MOT?

An illuminated engine management light is a Major defect at MOT since 2018 — an automatic fail until the fault is repaired and the light stays off. Clearing the code without fixing the cause is not a repair.

Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.