Abstract: An apparatus for testing an internal combustion engine is disclosed. The apparatus comprises a probe for electrical connection to at least one spark plug wire of the engine being tested. A peak hold circuit operates to store a representation of a peak firing voltage of the rectified secondary ignition signal and attenuates the stored representation at a predetermined rate over time to enable measurement of peak firing voltage. The testing apparatus is connected to a voltmeter to display a value of the peak firing voltage. Alternatively, the testing apparatus includes a display for displaying the value of the peak firing voltage. Preferably, the testing apparatus is housed in a housing of a size to be held in a user's hand, with a power supply contained therein.