I don't know what the mas sensor is>
I would think that you have a problem with the air injection not working properly. I would start by checking for a vacuum leak to the solenoid that actuates the valve in the air injection. This is critical for it to work.
Also, there is a test sequence that can be done to verify its operaiton. It is outlined ( the full test) in the 124 CD-1 under the group 14 in the Emmision control catagory.
Good luck We sell the CD's if your tech doesn't have the information needed.
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