n0ah

Car has been sitting for a few months, never had problems with this before. First drive after charging battery and replacing tire, the tachometer is going all over the place. Sometimes the tach reads normally, and sometimes it trips out while the car runs fine. The actual revs are a bit jumpy at times but not enough to slow the car down. If I let off the accelerator and go again it usually fixes it.

Now the car is reporting a P0725 trouble code, and other threads on here suggest it's the ignition coil because of some resistor in the v6 models.

What would be the best approach to diagnosing this issue? I am not a car guy but I am a broke guy so I can figure stuff out and have tools, multimeter, OBD2 reader + live data, etc. Usually I used Chilton Library to figure these things out but my library removed access.

Also, regarding the ignition coil: these other threads keep saying "the" coil rather than "a" coil. I was under the impression that cars have 1 coil per cylinder, is that a different thing in this case? The coil is $50 at AutoZone, do I need one or six? If it's one how would I know which one is causing the problem..