Think of incomplete combustion like a stove flame turning yellow instead of blue; the yellow flame indicates not enough oxygen is mixing with the gas, producing soot and dangerous carbon monoxide instead of clean heat.

Incomplete combustion occurs when there is insufficient oxygen ($O_2$) to fully oxidize carbon atoms in fuel into carbon dioxide ($CO_2$). Instead, carbon reacts to form $CO$. The chemical process is represented as $2C + O_2 \rightarrow 2CO$. Atmospheric $CO$ reacts with hydroxyl radicals ($.OH$), which would otherwise destroy methane ($CH_4$), effectively extending the lifespan of potent greenhouse gases.

In competitive exams, remember that Nitrogen ($N_2$) and Oxygen ($O_2$) make up 99% of air but are not greenhouse gases because they are homonuclear diatomic molecules with no dipole moment change during vibration.