A solenoid is determined abnormal if a current flowing therethrough at a certain duty ratio falls within an abnormal range. After the abnormal determination, a first duty ratio is used to cause a current to flow through the solenoid at such a current value within a first range as to not drive the solenoid valve. The solenoid is determined provisionally normal if the current at the first duty ratio falls within the first range. After the provisionally normal determination, a second duty ratio is used to cause a current to flow through the solenoid at such a current value within a second range including a current value range greater than the first range as to drive the solenoid valve. The solenoid is determined normal if the current at the second duty ratio falls within the second range.