# What is responsible for the smell of incomplete hydrocarbon burning and what sensor can detect it?

After I pass the smoke from partially burned fuel oil through a filter, I can still very strongly detect the smell. It is the same thing you smell from the exhaust when an old car or truck goes by.

Even though I can easily smell it with my nose, none of my sensors can detect it. I've tried $$\ce{VOC}$$, $$\ce{O3}$$, $$\ce{PM_{2.5}}$$, $$\ce{CO}$$, $$\ce{NO2}$$, & combustible gas detectors and all come up clear on the filtered exhaust.

So what is left in this gas that is responsible for this strong smell (and the resulting headache) and what sensors can I use to detect it?