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A Constant Volume Sampler (CVS) over dilutes the exhaust gas sample when testing Plug-In Hybrid Electric Vehicles (PHEV). This is because the CVS continues to fill the sample bag when the engine is shutdown. With a PHEV, it is possible to complete an FTP test with the engine running less than 20% of the time, resulting in a CVS bag dilution ratio in the range of 100 to 300. The CVS dilution ratio should be in the range of 5-25 for accurate results. At higher dilution ratios, the gas concentrations of CO, NOx and THC approach the ambient background level in the test cell. At a dilution of 100, the CO₂ concentration in the sample bag is about 0.13%, which is only 3 times the air background concentration. The measurement errors caused by over dilution create errors of 10% to 30% in the calculated mass of CO₂, CO, and NOx. Estimated errors for THC are in the range of 200%.