Honda Pioneer 1000 2017 Owner's Manual page 280

The combustion process produces carbon monoxide (CO), oxides of
nitrogen (NOx) and hydrocarbons (HC). Control of hydrocarbons and
oxides of nitrogen is very important because, under certain
conditions, they react to form photochemical smog when subjected
to sunlight. Carbon monoxide does not react in the same way, but it
is toxic.
Honda Motor Co., Ltd. utilizes various systems to reduce carbon
monoxide, oxides of nitrogen and hydrocarbons.
The exhaust emission control system includes an oxygen sensor, a
three-way catalytic converter and a PGM-FI system.
No adjustment to this system should be made although periodic
inspection of the components is recommended.
The exhaust emission control system is separate from the crankcase
emission control system.
The engine is equipped with a closed crankcase system to prevent
discharging crankcase emissions into the atmosphere. Blow-by gas is
returned to the combustion chamber through the air cleaner.