Ahhh! The MAS relay. This is a power relay that combines the functions of the fuel pump/kickdown relay with the Klima (compressor control relay). It also adds the load of the air injection clutch coil. Basically it is the current carrier for all the heavy current devices related to engine management.
Its rather hard to evaluate that code as sceondary air injection is only used on cold motors. Whether it is working is evaluated by the ECU using the O2 sensor. Your tech is covering all the bases it seems. He has replaced the sensor used for evaluation, he has replaced the temp sensor I presume because it would be involved with when the system would be activated, and the MAS would be replaced on the concept that the AIR pump clutch wasn't getting electrical activation.
The AIR pump pumps when its clutch is electrically engaged. It blows through a valve that opens under vacuum control. The hoses for air flow and vacuum control must be tight (no leaks). There are two valves between the AIR pump and the exhaust. One acts as a check valve keeping exhaust from going up into the pump and the other checks air from being aspirated into the exhaust after the pump quits. These parts all need to be checked.
Do you live in California? (bg)
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician