What are typical signs of a thermostat stuck closed or stuck open, and how does each condition affect engine temperature and emissions?

• A. Stuck closed causes overheating; stuck open causes extended warm-up and cooler operating temperatures; both degrade efficiency and emissions and may cause overheating or poor cabin heat.
• B. Stuck closed causes rapid cooling; stuck open causes instant overheating.
• C. Both conditions improve engine performance.
• D. Neither condition affects emissions.

Answer: (A)