Closed loop operation depends on feedback from the exhaust oxygen sensor. When the O2 sensor is heated to its operating temperature and actively switching between richer and leaner voltages, the engine control module uses that signal to continuously adjust fuel metering and keep the air–fuel ratio at the stoichiometric point. That switching signal is what signals that feedback-based control is active. If the oxygen sensor isn’t switching, the ECU can’t use its feedback to trim fuel, so the system would be in open loop. The MAP sensor reading atmospheric pressure or the MAF sensor simply being warmed up aren’t, by themselves, the condition that defines closed loop. Open loop is the state opposite to closed loop, where no sensor feedback is used for fueling.

The test uses a mid-range engine speed to reliably observe the EGR valve’s behavior when it’s commanded to open. At about 2000 RPM the engine is under enough load for the EGR system to be active and for the valve’s effect on the intake and combustion to be detectable, but not so high that other variables overwhelm the reading. Idle conditions don’t provide a consistent opportunity to see the valve opening because EGR flow is minimal and the system can be lazy or contradictory at low load. At very high RPM, the engine control may shift to different behavior and the test becomes less representative of normal EGR operation. So about 2000 RPM is chosen because it gives a stable, observable condition to verify that the EGR valve opens and flows correctly.

The test checks when you should use the Smog Check approved fan to ensure the engine can be cooled properly during the test. It is used only when the ambient temperature is above 72°F, because hotter conditions make it harder for the cooling system to maintain stable engine temperature and prevent heat soak, which could skew emissions readings. In cooler weather, natural cooling is usually sufficient, so the fan isn’t needed; using it then could alter readings, and never using it in hot weather would risk overheating and invalid results.

In the Smog Check Program, a Grey Market Vehicle is one that was originally manufactured for sale outside of the United States. This matters because those cars were not designed to meet U.S. (and California) emission standards, so they often lack the required emissions equipment or calibration. As a result, they may need modifications or a special exemption to pass smog certification, or in some cases may not be eligible for standard certification at all. The other descriptions describe off-road use, alternative fuels, or U.S.-market origins, which do not define a grey market vehicle.