What is the relationship between logos and fallacies?

No relationship between logos and fallacies

"Fallacies are common errors in reasoning that will undermine the logic of your argument" 
Basically, they're errors found in reasoning that weaken the logic in your argument.

Logos "appeal to the audience's logic and reasoning"
An example of a logo is considered, "An ad that shows the features and strong points of a product"

You can infer that logos can be found in fallacies. Fallacies rely on faulty logic, which can make logos less effective. 

**Therefore, the relationship between logos and fallacies is likely that logos are the logical and reasonable ideas that appeal to your audience while logos in fallacies prove to be less effective because they have false information.