What is the recommended procedure for checking the engine and tailpipe emissions?

The recommended procedure for checking engine and tailpipe emissions involves assessing the vehicle's performance under different conditions to get a comprehensive understanding of its emissions profile. By checking twice, once at high speed and once at low speed, the inspector can evaluate how the emissions vary with engine load and speed.

High-speed checks typically reflect the emissions during highway driving, where fuel efficiency and engine performance can differ significantly compared to low-speed operation, which often simulates city driving conditions. This dual testing approach helps in identifying any anomalies or issues that may only present themselves under certain operating conditions, ensuring that the vehicle meets emissions standards effectively across a range of driving scenarios.

Other methods, such as checking only at low speeds or randomly during a driving session, may not provide a full picture of the vehicle's emissions behavior under varying conditions, which is why they are not included in the recommended procedure.