We have developed a Scanning Kelvin Probe Microscopy (SKPM) based method to probe the effects of low-mobility minority carriers on the switching characteristics of organic field effect transistors (OFETs). By measuring and modeling the transient potential changes in the OFET channel after an applied gate bias pulse, we can extract the low mobility of the minority carriers and understand how these carriers play a key role in the device operation, in particular, in the screening of the gate potential in the OFF state of the transistor and in the recombination of majority carriers trapped in the channel after an ON state stress.