As newer homes are being built tighter than the existing housing stock, questions have been raised about the concentrations of pollutants of concern in new homes and how mechanical ventilation systems can address this issue. This study measured pollutants of concern in 70 new homes with mechanical ventilation in California, USA and compared the results to a previous study of home without mechanical ventilation. The key pollutants were measured using both time-integrated and time-resolved over a one-week period and included formaldehyde, PM2.5 and NO2. Each home was tested for air flows of mechanical systems, together with house envelope and forced air heating and cooling duct leakage. The results show that the homes complied with dwelling unit ventilation fan flows and most of the time with kitchen and bathroom requirements. The measured pollutant concentrations were almost all within acceptable limits and showed that the installed ventilation flow rates (that complied with California building standard and ASHRAE 62.2 requirements) provided acceptable indoor air quality. The mechanically ventilated homes had more consistent ventilation, resulting in less extreme pollutant concentrations. However, there remain issues with system operation, e.g., poor labeling of easily accessible controls led to three-quarters of the dwelling unit ventilation systems being turned off when homes were first visited for this study. This paper summarizes the results of the diagnostic testing and time-integrated field measurements, together with implications for ventilation standards.