Discuss it with your neighbor?

Odors aside, I would be concerned about CO2 coming from a garage. Plus, is this fire code? Am not sure what you mean by the wall extended to the floor. Perhaps a detector and if it checks out, a very good reason to consider resolving the issue.

you don't say what the odor is or the potential source.

A wall separating two dwelling units must be fire rated and a fire rated wall must extend from the lowest level to the roof. If that is not the case the designer/builder is at fault. This condition must be repaired for life safety as well as to eliminate odors.

Find the original construction drawings at the building department and see what was proposed vs what was Just because it passed inspect doesn't necessarily mean it was built correctly.

Are you saying that you share a double garage that is divided by a partial wall?

" I would be concerned about CO2 coming from a garage"
Just to clear up the chemistry, engine exhaust does contain CO2, carbon dioxide. Contrary to the news hype, I do not consider carbon dioxide to be dangerous. You expel some every time you exhale.
CO, carbon monoxide, is quite dangerous to warm-blooded creatures.It results from incomplete combustion of fuels containing carbon. All engines burning carbon-based fuel will emit some CO. I do not consider it to be poisonous in the typical sense, but when breathed in, it binds firmly to the hemoglobin in the blood and the ability of the blood to carry oxygen is thus reduced. At some point, the oxygen supply to the cells can become too low to sustain life.
If death does not occur, the body very slowly breaks down the the affected hemoglobin and the waste products of that are eliminated.

CO is odorless but if the odor is coming from a car exhaust the source should be obvious.

How do you know the wall is not continuous to the floor deck above?

This post was edited by Renovator8 on Mon, Sep 8, 14 at 23:00

CO2, the harmless (and helpful) carbon dioxide, and CO, the dangerous carbon monoxide are both odorless. CO is heavier than air and tends to collect in low areas. A gap under the wall is not good at all.

This post was edited by bus_driver on Tue, Sep 9, 14 at 7:28