Volvo Repair: 1991 Volvo, flash flash flash, example 121

Question
I have a 1991 Volvo 740 wagon.We put the key in the ignition and turn it and all it does is turn over.The car will not start.  It has been towed to the repair shop(the towtruck operator couldn't get it started) but when it was towed to the garage, it started up. They kept it there for 2 days and tried to start it up at least 30 times and each time it did start up right away.Once again, it sits in the drive way unable to start.  This has been over a period of 30 days. The repair guys are stumped. Can you help? Much thanks in advance!

Answer
first check for codes as described below.in the engine bay on the drivers side near the strut tower there is a small black box with a wire attached to it.place the wire in the terminal marked 2.turn ignition key on (not started)press the small button for 1 second and the code will be displayed as a series of flashes.a red led displays the codes.the code will be three digits.small pause in between flashes for a single number long pause between flashes for the next number.example 121 will be flash.....flash..flash.....flash.repeat three times or until first code repeats itself.to erase the code after all codes have been checked hold the button for a good 5 seconds then release the light should light steady,then press for another 5seconds.this will erase the codes.  email me the codes and i will tell you what they are..socket 2is the fuel system 6 is ignition.