Who is the governing body with respect to engineering ethics?

It depends on the country where the engineer is practising. In Canada, there are 12 different provincial/territorial associations that regulate the engineering practice, with all of these associations respecting guidelines outlined by another governing body called Engineers Canada. In order to call oneself an "engineer" in Canada, one must be fully registered with one of the aforementioned associations. In the USA, there is a similar arrangement, but I'm not sure exactly how it works (i.e. I don't know if there is a governing body in every state, nor do I know if an engineering society like the SAE or IEEE would enforce ethics). An important distinction between the USA and Canada is that in the USA the Word "engineer" can be used much more loosely - for example, the term "Microsoft Certified Systems Engineer" is somewhat illegal in Canada. In the UK, the government does not regulate the engineering profession per se, but most engineers there will voluntarily join a technical society and thus gain the title of Chartered Engineer. Obviously, the main point is that there is no single governing body w.r.t. engineering ethics, so location and type of engineer definitely matter.