California requires most vehicles, including heavy-duty trucks, to undergo smog checks periodically to ensure they meet state emission standards. While there may have been instances in the past where certain heavy-duty trucks were exempt from smog checks due to specific regulatory circumstances or program implementation timeframes, as of my last knowledge update in September 2021, all heavy-duty trucks registered in California were subject to smog inspections.

Smog checks for heavy-duty trucks typically involve testing the vehicle's exhaust emissions to measure pollutant levels such as hydrocarbons, carbon monoxide, and nitrogen oxides. The specific frequency of smog checks for heavy-duty trucks may vary depending on the vehicle type, age, and location within California, but they are generally required every two years or annually for older vehicles.

If you own or operate a heavy-duty truck in California, it's important to stay informed about the state's smog check regulations and ensure that your vehicle undergoes the required inspections. Failing to comply with smog check requirements can result in penalties, fines, or even vehicle registration suspensions.

For the most up-to-date and accurate information on smog checks for heavy-duty trucks in California, I recommend checking with the California Air Resources Board (CARB) or your local Department of Motor Vehicles (DMV) office.