However, we did not spot any rigorous testing to verify that claim, so for now, consider this to be a standard photoelectric unit. Likewise, expert testing of the CO side of things is limited. No one has rigorously tested the CO detector performance of the second generation model. Consumer Reports says that the first generation Nest Protect worked well as a CO detector, though it was more effective in sounding an alarm when it spots high CO concentrations than low. User reviews, once mixed, are now much stronger as the product has been updated and tweaked since release. It currently earns a 4.