When the engine control module begins to fail, a car may have difficulty starting, the "Check Engine" light may come on, the gears may have difficulty shifting, fuel efficiency may decline and the car's engine may run poorly. If you do not replace the module, the symptoms continue and eventually the car fails to start.
Transmission problems cause many of the same symptoms as a failing engine control module, so mechanics should first rule out this possibility by checking transmission fluid levels. The engine control module, a circuit board containing the vehicle's central processing unit and flash memory, controls engine and transmission operation, managing several fuel and emissions systems to ensure appropriate exhaust levels, correct gear operation and maximum fuel economy. When the module begins to fail, it operates in a back-up mode that allows the engine to function, though performance is compromised.
In addition to replacing, repairing or reprogramming the engine control module, the mechanic must determine what caused the failure to prevent reoccurrence. Electrical surges due to shorts in components the modules control commonly cause the modules to fail. Mechanics should check the condition of the vehicle's fuel solenoids and ensure that wires are not corroded and that the battery is grounded properly.