Bad left motor field ciruit.
check the drive motor for shorting. chech the H-bridge card. if this is in a damp enviroment i have seen the 8 diodes on the H-bridge rust away Could be the logic card i have not seen many of these fail. but possible. could be the cables under the battery where they go under the bulkhead i've seen them wear there. make sure you are grounded when trouble shooting as these trucks are are static sensitive and can take out other componets when handling them good luck