Abstract: An internal combustion engine system having an internal combustion engine, a fuel source, a fuel filter for filtering fuel from the source to the engine, and a fuel pressure switch disposed between the fuel filter and the fuel source. An additional sensor is provided for sensing a condition of a substance fed to the engine. An output device is connected to both the fuel pressure switch and the additional sensor for indicating when either the fuel pressure switch indicates an improper fuel pressure condition or the additional sensor indicates the sensed condition is an unwanted condition.