A method of monitoring an onboard diagnostic system for a plug-in hybrid electric vehicle includes incrementing the denominator of an N/D ratio for the onboard diagnostic system only when a total time criteria, a vehicle speed criteria and an idle criteria are satisfied after an internal combustion engine of the vehicle has been fueled. The diagnostic system performance is summarized into a single N/D ratio. When an underperforming ratio is identified, the system controls the engine to provide more engine operation and subsequent diagnostic observability. The denominator of the N/D ratio is compared to a verification denominator to identify vehicles that are typically operated in a manner such that an engine-on cycle does not begin, or is not identified, until very near the end of the drive cycle, thereby preventing the denominator and a numerator of the N/D ratio from incrementing, and thereby providing a false passing performance ratio.