A sensor (16) is connected to a high pressure line (13) in a fuel injection system for an internal combustion engine. The sensor (16) produces an output current which is proportional to the pressure in the line (13). The current is converted into a corresponding voltage and a displayed a voltmeter (21). An alarm is produced when the voltage exceeds a predetermined value or range. A second voltmeter (23) is energized to display the voltage when the alarm is produced.
