On-board diagnostics of a diesel engine are very important in keeping emissions low and in maintaining the engine properly. Combustion sensing is the most important objective in engine diagnostics. SwRI-developed Temperature Controller for Glow Plugs (TCGP) can be used to detect the temperature in each combustion chamber of a diesel engine if it uses glow plugs. If one or more of the cylinders show lower temperature than the predetermined value at a given load/speed, the cylinder(s) is experiencing misfiring, improper fuel metering or other problems. The heating element in a glow plug can be used as a temperature sensor while heating the glow plug by a time-sharing method. The heating power can also be turned off completely so that the TCGP works only as a temperature measuring device. Thus, the TCGP or even a simpler glow plug sensing circuitry alone will work with the on-board diagnostics module in a diesel engine.