2005 Mitsubishi Endeavor Speed Sensor after Head Gasket Rep

My oil light was rapidly flashing when stopped and would go off after accelerating. I went by Jiffy Lube to have the oil changed but they were out of my filter so just topped it off but said it wasn't low. The next day on my way to work my car overheated and I blew a head gasket. My husband did part of the work himself but we took it to a mechanic in the end. We have had both heads revalved, new water pump, timing belt, and theromostat. The mechanic has fired up the car and said the engine is running fine but the car is going into limp mode when he takes it for a drive and the code is coming back as a speed sensor. He thinks it may be wiring somewhere but is waiting on advice from a transmission guy. Please help! I have been without a car for almost two months and have lost most of my patience with the mechanic. Should I take it to the dealership?