Keyless Operation Key Battery Check
WARNING: This page is about a different car, the 2008 Mitsubishi Lancer. However, it is still accessible from the selected car via links, so may be relevant.
Measure the voltage of the battery. If the voltage of the battery is lower than the standard value, replace the battery.
Standard value: 2.5 - 3.2 V
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.