Pin out ECM diagnostics for 2007 Ranger P5031 code after PCM replacement

You are correct, the ignition uses one power feed (Purple wire) and three trigger wires Yellow/gray, White and a Blue/brown. In your case the Blue/brown wire is the trigger for the 1-5 coil. The ECM is programmed to read the current ramp that fires the coils and monitors it based on each cylinder firing. So when it fires the 1-5 coil it looks at which cylinder should be in position to fire and produce power and reads the return pulse to determine if the coils fired. If the coil returns a pulse different than expected you get the P0351 code because on that engine that code covers both primary and secondary ignition. That can be caused if there is an internal cylinder issue (lowered compression or poor fuel mix) or if something like the plug wire or plug are not firing because of a failure, like carbon tracking on a cracked plug or a damaged plug wire. Very rare for it to be the coil itself unless the tower for a wire is damaged. Because the system is waste spark and you are not seeing anything about cylinder 5 as well, it isn't the PCM or the wiring to the coils that is the issue, or you would have a 2 cylinder misfire (1 and 5). As you only have the code for cylinder one then I would look at the plug wire and plug for that cylinder. See if there is some damage to either one. I have seen this code for plugs that were gapped improperly or if they were damaged (commonly carbon tracking)