Hybrid cars contain two batteries: a 12-volt auxiliary battery that powers the computer and accessories, and a larger traction battery that supplies the electric motor. If the small battery dies, the car may fail to reach its READY state, showing dim lights and unresponsive locks, though many models can be jump‑started at a designated terminal.

A failing traction battery triggers warnings such as "Check Hybrid System" or "Power System," reduces electric assistance, limits acceleration, or may shut the system down entirely. Overheating or blocked cooling vents can mimic these symptoms, and Toyota and Honda attribute such issues to obstructed air intakes or vents, respectively.

When a warning appears, read the dashboard message before acting. If it instructs a safe stop, pull over, turn off the engine, and call roadside assistance. Never touch high‑voltage cables or open the traction‑battery case; only trained technicians should perform high‑voltage work.

Diagnose with a professional scan, check cooling, wiring, sensors, and the 12‑volt charging circuit. A simple fix may clear a blocked intake, while more serious faults could require battery replacement. Verify warranty coverage—Toyota, for example, covers traction batteries in 2020+ hybrids for 10 years or 150,000 miles—before approving costly repairs.