My fast key sometimes is not recognized if in my left pocket when I get into my Mirage so I figured the battery may be at fault.
Found this testing procedure and did it myself.
Turn the multimeter on, and set it to "DC Voltage" mode.
Touch the "red probe" on the side of the battery with a "+" sign and the "black probe" to the other side.
Read the voltage displayed on the multimeter
Tips & Warnings
The CR2032 is a 3-volt battery. If the measured voltage reads less than 10 percent of the rated voltage, then the battery needs to be replaced. For the CR2032, that voltage would be anything less than 2.7 volts."
-quoted from http://www.ehow.com/how_7287405_test...2-battery.html
They come with a Pannasonic lithium 2032 and mine was at 3.01V still good to go!
Here is some battery removal videos fast key and standard key fob