Wrench Light Periodically Comes On.

The light you are referring to is turned on by the power-train control module in your car when it notices a problem which affects  normal engine or transmission operation, but which is not causing increased  exhaust emissions at the moment.  There are many different possible causes for  this light to be turned on; some possibilities are a failing sensor somewhere on  the engine or transmission, a sensor signal which is outside normal operating  range, a mechanical fault developing, an open or grounded circuit in the engine  control system somewhere in the vehicle, a vacuum leak, a controlled device such  as a solenoid or relay that is not responding to commands from the PCM,  etc...    As you can see there are literally hundreds of  possible failures that can cause this light to be turned  on.   To make the light go out and stay out  the root cause of the problem will need to be diagnosed and repaired. When the light is turned on, the computer  has also stored some clues in memory to indicate the general area that the  problem is originating from.   These clues include fault codes  that indicate what system(s) or circuit(s) are behaving abnormally,  as well as a frozen snapshot of the data stream coming into the PCM at the  moment the problem first occurred.   This information does not  identify what exact component or electrical connection is at fault, but gives  the technician a starting point to begin test in the vehicle to identify the exact  cause of the  problem. 

 To  correct the problem, the technician will first need to connect a scan tool to  the vehicle's data connector to be able to access this stored  information.    For each fault code stored, the manufacturer  has a very specific detailed test sequence to perform to the system involved to  isolate the cause of the fault.   Then, the affected component or  circuit will need to be repaired and the operation checked to verify that the  problem is gone. Without having the required  test equipment, there really is not much you can do at home in the driveway to  diagnose or correct this type of problem; for that reason you will need to take  it in to a repair shop to have the problem diagnosed.    If  you Do happen to have a professional level scan tool I can attempt to assist you  with preliminary diagnosis of the problem; however if you do not, it is far  cheaper to pay $100 or so for a professional diagnosis rather than to invest several thousand $$$ in the  equipment  required to test the vehicle. Hope this helps!