Hi Steve, so my little Interro is not so obsolete after all!! Quite a shock to see that AD are flogging them. Interro are a Californian Company which was taken over by Omnitec a few years ago. This very good scope was the USAs "Tool of the Year" in the early nineties.More can be read about it on Omnitecs American website. As for your problem with coils, you need to use the resistive O2 lead which should have been included with the tool. If not, try Ad to buy the lead.You will have to set the probe to read O2, otherwise the oscilloscope trace will be too small. You are correct in thinking that you are causing the misfire, indeed, if you were working on a single coil conventional distributer system, the engine would have stopped as soon as you touched the probe on the negative terminal. Any more info required, just ask, regards, Gwyn