It's easy to be fooled by CO2. Odorless, invisible, a benign part of every breath--what harm could possibly come of pouring more of it into the atmosphere?
According to my notes from a talk at Princeton University by Michael Oppenheimer, the extreme heat of Venus, and the extreme cold of Mars, are not so much a matter of their distance from the sun but of how much CO2 is in their atmospheres. Venus has a very high concentration of atmospheric CO2, while the concentration on Mars is very low. A quick check on the internet yielded the following: Venus has temperatures approaching 900 F, while the surface of Mars rarely gets above freezing.
In other words, the amount of CO2 in the atmosphere has everything to do with whether a planet is too hot, too cold, or just right.