An automatic transmission is arranged in a hybrid driveline employing an EV mode and an HEV mode, and establishes transmission ratio positions by engaging or disengaging frictional elements. A system or method includes: determining, based on a vehicle deceleration and a relationship between a requested transmission ratio position and an actual transmission ratio, whether or not interlocking is occurring, when the automatic transmission is in a state out of transmission shifting, wherein the interlocking is due to erroneous engagement of one of the frictional elements; and inhibiting the determination about interlocking, during a period from start of slip of a first frictional element of the automatic transmission to confirmation of convergence of slip of the first frictional element, in a situation that the first frictional element is slip-engaged for starting the engine in response to a request for mode transition from the EV mode to the HEV mode.