Volvo Repair: Resetting check engine light, flash flash flash, example 121

Question
I have a 1990 240 DL Wagon which is running fine.  I took it for an annual inspection and oil change to a mechanic (just a tie rod replacemnt and parking light needed for inspection).  When it came back, the "Check engine" light was on.  Every few years this light comes on even when no problem is found.  I have only basic tools at home.  Can I read the diagnositic code with only basic tools?  How can I reset the light?  Thank you.  Rick

Answer
First, sorry for late response.I am in SW Florida and if you do not know we were just hit by  Hurricane Charley Friday and I just got power 30 minutes ago.

n 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.