Glow plug connector loose (Pro .15)
Examine the glow plug connector at the end of the wire. The heat from the engine can cause the connector to expand, loosening its connection with the glow plug. Use a pair of needle nose pliers to gently crimp the end of the connector until it fits snugly onto the glow plug.
If the connector on the blue wire is damaged, it can be replaced separately as Traxxas part #4581.