Buildup of creosote on the flue walls can re-ignite and cause fires. Tight seals and joints prevent leaks. Refer to parts list in your owner's manual. A broken thermostat can prevent air inlets and dampers from opening or closing automatically. The stove will be less efficient and more expensive to operate.
Thermostats control either primary or secondary air, or both.
Tampering may ruin the precisely designed secondary combustion capabilities, resulting in lower efficiencies, higher operating costs, and greater pollution.