During winter weather inspection, which system should receive extra attention?

Exhaust system leaks are especially dangerous when cab ventilation may be poor (windows rolled up, etc.). Loose connections could permit poisonous carbon monoxide to leak into your vehicle. Carbon monoxide gas will cause you to be sleepy. In large enough amounts, it can kill you. Check the exhaust system for loose parts and for sounds and signs of leaks. [California Commercial Driver Handbook]