The glow plugs generate an output voltage when the engine is running. They are acting like thermocouples. The combustion temperature burn is reflected in the output voltage. Usually about 11 milli volts at idle but depends on the individual engine and plugs. Some as low as 6.3 millivolts output per average plug for example. If something is abnormal in any cylinder it will be reflected in the plugs voltage output. We are finding that comparison between plugs is a good indicator of what is going on. So easy to do. Just disconnect the harness and read each voltage separatly at the plug. Right now we have been dealing with the early stages of development by trying to develop it as a superior method to time the injection pumps to the engine. Results look really promising so far.