The PHEV-ECU monitors the whole system state directly by monitoring sensor signals, actuator drive signals and EV components, and indirectly via controllers. If a fault occurs, the ECU will inform the driver of it, store the fault information including a DTC and status, and, at the same time, execute the corresponding fail-safe control. The stored fault information can be read out and erased using M.U.T.-IIISE. The PHEV-ECU supports the following diagnosis functions: