MAC Cat. Conv.-NEED HELP!! Efficency Monitor?? ASAP

[Jonesy111]

'98 3.8 V6 Mustang- Currently has a MAC Cat Coverter on it with Spin Tech Muffler side pipes. Check engine light keeps coming on...is this a problem? I have tried replacing the 2 downstream O2 sensors but still the light is on. Ford diagnostic came back with trouble code for the Cat Efficency Monitor. There are suppose to be 2 monitors on the bottom that go with the O2 sensors correct?...one on the right and one on the left if I am not mistaken. Please tell me if I am wrong...but if not Firestone is in trouble because I only have the one on the left now because they did not put it back on when they changed the O2 sensor. Is there anywhere I can get another Efficency monitor for this MAC??? PLEASE HELP!!! I'm a 20yr old young lady who is a first time mustang owner still learning the ropes...PLEASE PLEASE help if you can!!! Oh yeah PS is it a problem if the diag came back with the Mass Air Flow Censor with a 0% read out??? Thank you for your help!