During the idle test in a two-stage tailpipe test, what is the vehicle's engine status?

In the idle test of a two-stage tailpipe emissions test, the vehicle's engine is required to be at idle. This means that the engine should be running but not accelerating or revving. The purpose of this stage is to measure the emissions of the vehicle when it is operating under normal idling conditions, which is essential to determine how well the vehicle meets emissions standards while not under load or additional power demands.

Other options, like running at high RPM or revving, do not accurately simulate idling conditions, which are critical for this particular test phase. Having the engine off would eliminate any emissions readings altogether, while revving the engine would cause it to produce higher levels of emissions, which would not provide the valid idle emission readings that the test aims to obtain. Therefore, having the engine at idle is integral to achieve the right conditions for testing emissions effectively.