That is a good question.
The MAS unit does quite a few things
See this document, PDF age 29 or so (page on the image is "108/8")
http://www.startekinfo.com/StarTek/outside/11832/Resources/201Create/PDF/01002.pdf
-Oxygen Sensor Heating
-Oxygen Sensor Signal
-Fuel Pump Control
-Air Pump Control
You might find something in here.
Untitled Document
If your problem is like my 420SEL, the control unit was doing its just most of the time so a test likely wouldn't reveal the problem. The cold solder joint would only act up 1% of the time. It likely wont act up during a test. The only way I was able to figure out what was wrong with it was to open up the box and visually inspect the joint.
I have a MAS unit here on my table. I might open it up at some point here and post a thread with pics. It looks like it opens up fairly easily. Looks like you gotta take off the metal surround somehow and then there is just a bead of silicon acting as a seal.
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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
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