engine

Is it the exact same engine, year--make--model? Where applicable, what about your security system? If it cranks good but won't start, have a helper crank it, while you check for spark at the plugs. If you have spark everywhere, use a gage and check fuel pressure.

Can you communicate with computer, check for applicable trouble codes?

yes, same engine and year model, the car, cranks but it want'n start, and them check engine light flashes like ten times and a code dobe and p1684 shows up.!!

Below are possible causes for that code and the flow chart.