I'm as big on DIY as anyone, but this is a case where I would take it to the dealer and pay for the diagnostics. The reason is that the diagnostic equipment is so expensive and the odds that I would ever use it again so remote. This becomes the least expensive option in the long run. We all know how "cheap" guessing and swapping parts can get.
First get a quote for complete diagnostics from the dealer or independent shop that specializes in Honda's and then determine if you think it is cost effective to do it and keep the car. I would not use book value of the car as a guideline but the value of the remaining expected life of the vehicle minus any upcoming large scheduled maintenance items, i.e. timing belt service, tires, cv joints etc.