This is always a problem in the professions and "knowledge class" discussion. Engineers apply a body of knowledge to problems in a way that can not readily be routinized. This is the starting point for considering a profession. Yet they are often the outliers in other cultural measures of professionals.
Alvin Gouldner, in The Future of Intellectuals and the Rise of the New Class made a distinction between "intellectuals" and the "technical intelligentsia." They differ in the kinds of knowledge they use. They are both part of the "new class" - what later came to be called the "knowledge class" - because they both make their living from knowledge that is essential to running the economic system. That is the Marxist part of Gouldner's determination of a class. They are also both part of the knowledge class because they rise to the defense of knowledge and reason when they are under attack. When the thugs burn books, the engineers join the barricades. When rationality, or even science, are under attack, engineers and cultural specialists stand shoulder to shoulder.
I think what unifies all professions is that the body of knowledge they must apply is vast, so applying the right bit of knowledge requires judgment. What unifies most professions is that they apply knowledge to people, and people are infinitely various. A vast body of knowledge applied to varied people makes for lots of judgments. Engineers apply a vast body of knowledge, too. This requires judgment. But for the most part, the problems they apply that knowledge to are not people. That makes their knowledge work different in kind from that of most other professionals.
Nonetheless, I am counting engineers as professionals.