Coil on plug ignitions work just like any other kind of ignition. They just make do without plug wires leading from the coil output to the spark plug. COP ignitions rose in tandem with electronic fuel injection and computers fast enough to provide spark information in real time to multiple coils. Testing a COP coil is simple, requiring only the most basic of hand and electronics tools.
Remove the car's engine cover and unbolt the coil assembly from the valve cover. Unplug the wiring harness leading to the coil, noting, if possible, the locations of the red and white, or dark and light, wires. The red or light wire should correspond to the positive terminal. The black or dark wire should indicate the negative.
Set your digital multimeter to read single ohms of resistance (indicated by the Greek Omega symbol) in the single ohm setting. Testing resistance in the coil will tell you if it's experiencing an internal short or open circuit that will impede function.
Place your red probe tip on the positive coil terminal and your black probe lead on the negative terminal. Manufacturer specifications will vary, but resistance across these terminals should read in the single digits or lower, probably below 1.0 ohms. If you get a reading in the hundreds of ohms or higher, then the coil is shot.
Set the multimeter to read in thousands of ohms. Poke the black probe tip up into the coil output where the spark plug would normally go, and put the red probe tip on the positive terminal where it was for the last test. You should get a reading of 5,000 to 10,000 ohms (5.0 and 10.0 if you're reading it in thousands). Some "segmented" coils might read as high as 15,000 ohms. If you get a reading that seems suspiciously high or low -- 4 ohms or 40,000 ohms -- or your tester indicates infinite ohms, the coil is dead.
