P2504
Charging System Voltage High
- Severity
- High
- Driving
- Drive only to a garage
Charging system voltage is too high, measured at the charging system output. The charging system is pushing more voltage into the vehicle than it should. This is more damaging than it sounds: sustained overcharging boils the battery electrolyte, and the excess voltage stresses every electronic module in the vehicle - which can turn one cheap fault into several expensive ones.
Possible causes
- CommonFailed voltage regulator, usually inside the alternator
- CommonFailed alternator
- PossibleFaulty battery temperature sensor, so charging is compensated wrongly
- PossiblePoor earth between the alternator and the battery, confusing the regulation
- PossibleControl module commanding the wrong charging voltage
- PossibleAftermarket or incorrectly specified alternator fitted
What you might notice
- Headlights unusually bright, and brighter as revs rise
- Bulbs failing repeatedly
- Battery warning light on
- Smell of rotten eggs or acid from the battery
- Battery case swollen or hot
- Several unrelated electrical faults appearing at once
What to check, in order
- Measure charging voltage at the battery with the engine running and compare it with the manufacturer's specification. A reading well above it confirms overcharging.
- Check the alternator earth and the main charging cable connections. A poor earth can make the regulator over-compensate, and that is far cheaper to fix than an alternator.
- Check the battery temperature sensor if the vehicle has one - charging voltage is temperature-compensated on most modern vehicles, and a sensor reading wrongly cold makes the system overcharge.
- Look at the battery. A swollen case, a hot battery or an acid smell means it is being cooked and should be treated as urgent.
- Do not keep driving with a confirmed overcharge. Every mile risks another module.
Read this code from your own car
Plug an ELM327 adapter into the OBD-II port under the dashboard. Pin16 reads every stored, pending and permanent code, explains each one like this page does — offline, with no account — and shows what the car recorded when the fault was set.
Where this code sits
P00xxP01xxP02xxP03xxP04xxP05xxP06xxP07xxP08xxP09xxP0AxxP0BxxP0CxxP0DxxP0ExxP0FxxP10xxP11xxP12xxP13xxP14xxP15xxP16xxP17xxP18xxP19xxP1AxxP1BxxP1CxxP1DxxP1ExxP1FxxP20xxP21xxP22xxP23xxP24xxP25xxP26xxP27xxP28xxP29xxP2AxxP2BxxP2CxxP2DxxP2ExxP2FxxP30xxP31xxP32xxP33xxP34xxP35xxP36xxP37xxP38xxP39xxP3AxxP3BxxP3CxxP3DxxP3ExxP3Fxx
Vehicle speed control, idle control and auxiliary inputs
P2500P2501P2502P2503P2504P2505P2506P2507P2508P2509P2510P2511P2512P2513P2514P2515P2516P2517P2518P2519P2520P2521P2522P2523P2524P2525P2526P2527P2528P2529P2530P2531P2532P2533P2534P2535P2536P2537P2538P2539P2540P2541P2542P2543P2544P2545P2546P2547P2548P2549P2550P2551P2552P2553P2554P2555P2556P2557P2558P2559P2560P2561P2562P2563P2564P2565P2566P2567P2568P2569P2570P2571P2572P2573P2574P2575P2576P2577