We need to order the spare parts, take apart your electrical system, fuel pump and engine and then put it back together again.
since there are many different parts here then why does the author use "it "in"put it back together again"here?THX.
I'm assuming the author is talking about a car? If so, in this case, the "it" is referring to the car. However it's strange that he would refer to the car as "it" in a sentence where he did not mention the car prior to using "it". So it might be the case that the author is referring to the electrical system, fuel pump, and engine when he said "it". But this is making a mistake. If he really is referring to those 3 parts, then it should be "... and put them back together again". In my opinion I think it is the latter, where he is referring to the 3 car parts and made a mistake by saying "it" instead of "them". Hope this helps!