Running the air conditioner and heater simultaneously in a vehicle won't directly damage the engine. However, it can put an additional load on the engine, which may lead to reduced fuel efficiency and increased wear on some engine components.
When you run the air conditioner, the engine must power the air conditioning compressor, which places an extra load on the engine. Similarly, the heater in most vehicles is powered by the engine's cooling system, which uses engine heat to warm the cabin. So, when both the air conditioner and heater are running, the engine has to work harder to provide power for these systems.
In modern vehicles, the engine is designed to handle such loads, and running both the air conditioner and heater occasionally should not cause any significant damage. However, there are a few points to consider:
Reduced fuel efficiency: Running both the air conditioner and heater simultaneously will increase fuel consumption, as the engine needs to work harder to provide the required power.
Engine wear: The additional load on the engine might contribute to slightly more wear over time, although this wear is generally considered negligible for modern engines.
Engine temperature: Running the heater may help maintain the engine's optimal operating temperature during cold weather, but running the air conditioner in extremely hot conditions can contribute to engine heat, especially in stop-and-go traffic or while climbing steep hills.
Battery load: Running both the air conditioner and heater may put extra strain on the vehicle's electrical system, potentially draining the battery faster.
In most cases, it's best to use the air conditioner or heater separately based on the weather conditions. If you find yourself needing both systems to maintain a comfortable temperature in extreme conditions, it's generally acceptable to do so occasionally. However, for everyday driving and to maximize fuel efficiency, it's best to use these systems individually when needed.