Which emission control system helps reduce HC and CO during engine warm-up?

• A. The Air Injection System (AIS) reduces HC and CO during engine warm-up.
• B. EGR reduces NOx during warm-up.
• C. TWC reduces HC during warm-up.
• D. The vacuum system reduces emissions.

Answer: (D)

During engine warm-up, the catalytic converter isn’t yet at its light-off temperature, so it can’t efficiently oxidize HC and CO. The Air Injection System addresses this by adding fresh air into the exhaust, which helps burn HC and CO as the exhaust passes by, reducing these emissions before the catalyst is fully effective. The other options don’t target HC and CO during warm-up as directly: EGR mainly lowers NOx by diluting the intake charge, the catalytic converter’s effect is limited until it’s hot, and a vacuum system by itself isn’t a direct emisiones-reducing mechanism.