What is the MG P0504 fault code?

[SanGabriella]

MG fault code P0504 indicates Brake Switch A/B Correlation (Signal Implausible). Possible causes: If this fault code appears, it may indicate a malfunction in the brake switch or a circuit fault. Generally, such faults are caused by a damaged brake light switch or related circuit open/short circuit issues, primarily due to switch failure or poor contact in the wiring harness connectors. Solution: If this fault code appears, it is necessary to inspect the brake light switch assembly, wiring harness and connectors, and the brake light switch signal circuit. This fault requires a technician to use specialized tools for diagnosis. Once identified, contact a professional technician promptly to inspect and resolve the issue.

[OKaden]

I've encountered the P0504 trouble code several times on MG cars. It's essentially an issue where the A and B signals from the brake switch don't match. You know brake switches typically have two signal circuits, right? One controls the brake lights on/off, and the other is related to the cruise control system. If the ECU detects a mismatch between these two signals - for instance, one indicates braking while the other doesn't respond - it will trigger this code. Common causes include problems with the brake switch itself, such as worn contacts or poor connection inside; wiring issues like loose connectors or aged/broken wires; or external interference causing signal confusion. Once I repaired an MG GT where the owner complained about cruise control failure. After reading code P0504, we found corroded switch connectors - replacing them solved the problem. Such issues should be addressed promptly because non-functioning brake lights increase accident risks. During daily driving, pay attention to whether your brake lights illuminate properly. Regular maintenance checks on switch components can prevent future problems.

[FaithDella]

Oh, P0504 is quite common on MG vehicles. Simply put, it indicates a brake signal malfunction. The A/B correlation refers to the two circuits of the brake switch not coordinating properly—one might be the brake light signal, and the other connects to the cruise control system. When they're out of sync, the ECU sets this code. The causes vary: a faulty brake switch, short or open circuits in the wiring, or even a blown fuse can trigger it. I once drove a friend's MG 5 with a similar issue. After checking online, we inspected it ourselves and found a loose connector—pushing it back in fixed the problem. If your cruise control suddenly fails or the brake lights flicker while driving, stop and check immediately—don't delay. Replacing the switch isn't expensive, but always use genuine parts to avoid bigger issues.

[Danielle]

Fault code P0504 indicates a mismatch between the two signals from the brake switch. In MG vehicles, this is typically caused by a faulty brake switch, damaged wiring, or fuse issues leading to signal desynchronization. When the ECU detects this, it triggers the code, which may be accompanied by cruise control failure or abnormal brake light behavior. I've personally handled this issue through DIY methods: first, inspect the switch under the brake pedal for physical damage; then use a multimeter to check if the resistance is normal; if it's a wiring issue, tighten the connections or replace the wires. Safety first—avoid using cruise control until the issue is resolved.

[MacCody]

P0504 indicates a brake switch signal inconsistency, commonly found in MG models. It involves the dual-signal circuit for the brake lights and cruise control system. If these signals conflict, the ECU will trigger the fault code. Possible causes include: switch wear (e.g., oxidized internal contacts), short or open circuits in wiring, or relay failure. During repairs, I often find this issue prevalent in aging vehicles. It's recommended to use a diagnostic tool to read data streams for pinpointing the problem. Replacing the switch typically resolves the issue.

[JoshuaLee]

I've seen quite a few P0504 issues on the MG forum. Simply put, it's a fault caused by mismatched brake switch signals. Signals A and B represent braking function and auxiliary systems respectively. When they're inconsistent, it triggers the error code. Common symptoms include cruise control failure or brake light flickering, posing significant safety risks. It's recommended that owners pull over promptly upon discovery, check the fuse box and switch, and replace parts if necessary. Getting it inspected at an authorized dealership is more reliable - the cost won't be too high but ensures safety.