My son's 98 C43 needs a new Air Mass Meter. His mechanic tells him not to DIY cause the computer won't recognise the new sensor. Is this true?

He has a code reader and has had the error (unequal fuel line pressure, I think) showing for a long time. He decided to finally fix it because a friend with an E500 accelerated much faster. Its going to cost him about $500 for the work, but he could do it himself.

Why wouldn't the computer look for a change in the sensor and update itself? And isn't that part of reseting the codes with the code reader?

Thanks