The laptop cooling fan runs all the time. It was turning on and off before but now it runs constantly.
The heatsink is clogged with dust.
Possible troubleshooting steps and repair solution:
Most likely the cooling fan runs all the time because the heatsink is clogged with dust and has to be cleaned.
A dust buildup inside the heat sink stops normal airflow and because of that, the CPU runs hot all the time. The extensive heat triggers thermal sensors and the cooling fan starts running.
Because of the dust buildup, the CPU temperature is always above the norm, and as a result, the cooling fan runs all the time.
Most likely you can fix the problem by cleaning the heat sink.