Question Poisonous fumes can enter the cab due to: Answers A defective exhaust system. The wrong type of fuel being used. A lack of oil in the engine. Under-inflated tires. Explanation Defects in an exhaust system are very dangerous because they can cause poisonous fumes to enter the cab or sleeper berth. Take a FREE Practice Test Now This questions appears in the following tests: Colorado Knowledge Test Class A , Massachusetts Knowledge Test Class A , Montana Knowledge Test Class A , North Carolina Knowledge Test Class B ,