Q&A

 Master

P0307 can be caused by many things i.e. a misfire in #7 cylinder. What I do is make sure the compression is good then switch the coil and plug from a known working cylinder then if the misfire changes to another cylinder, then you know you have a bad plug/coil. A resistance reading on a coil is almost useless, because, the coil usually fails by arcing internally, which you can not measure. You need a scope to see that.
By the way, at 100k, I replace all the coils and plugs ( and wires if equipped ) on Ford truck motors, they are usually gone by then.

 Master

As stated by the previous mechanic, misfire codes can be caused by several different concerns. Ford Motor Company is now recommending replacing the spark plugs at a lower mileage interval- 75k miles. We see the COP or the coil on plug as the most common concern causing a misfire. We have the latest technology to stress test all the coils, but this is very time consuming. One technician in our shop simply uses a long screwdriver in the coil boot and gap grounds it against a suitable location and examines the spark. Most failed coils will only give a small yellow spark like a lawn mower. A healthy coil will give a blue spark with a notable crack noise. Comparing two different coils-, one failed and one healthy and it is an easy test. Like the previous mechanic, relocating coils, injectors and spark plugs usually is a good method of isolating the failed component if the rudimentary coil test does not yield results. I hope this helps.