Here is trick to find a COP coil primary that is not getting fired it will work on Android phones that have magnetic field detector. --- some phones have this sensor some don't.
It works on my Motorola Moto , but not on my HTC 626 or my Kindle Fire Tablet.
You will need to load a free app from Google Play Store "Magnetic Field Detector by Mobile Develop Team" .

Once you have the app up and running by using a magnet you should be able to figure out which end of the phone the magnetic sensor is located. On the Motorola Moto G it is located near the bottom edge of the screen.

Now with the car engine running as you hold the phone over the engine the first thing you will detect is the huge magnetic field from the alternator but if you hold over an ignition coil you should be able to see the firing pattern of the coil primary --- you will get a stronger pattern if you do a snap throttle.
It is best not to actually touch the COP in case it has bad insulation on the secondary which could zap the phone.



It works much better if any engine covers are removed
This can also be used to detect if a relay is energised, I tried it on a high impedance Bosch fuel injector but the pulse signal is very weak.

[Edited on 22/6/16 by britishtrident]

very handy,...

Knowing my luck (and the quality of my ignition system at the moment!) my phone would become the equivalent of a lightning conductor, with me as the ground!!