MQ-7 Module Carbon Monoxide
The module on the link doesn't have PWM feature for heater. The LM393 is comparator, which fires DOUT HIGH when sensor output reaches certain threshold (set by potentiometer). So if you want to implement PWM, you have to drive it  "manually" with MCU PWM and MOSFET (you can't drive directly by MCU pin since datasheet says that the heater consumes 370mW or 70mA). IMHO it is expected, since when using heating cycles you have to read the sensor output say after heating cycle, not just anytime (to get more consistent measurements).

It is not clear from three page datasheet, but seems the heating cycle MUST be implemented. On the other hand, I wonder if such blind heating make sense without actual temperature detection/stabilization. Datasheet says operation temperature range is -20 to +50 deg C. Maybe heating is needed just to make sure that temperature is well over -20?