Took my 08 G35S Sedan in for the CA smog check, and it overheated during the test and thew a code.

It has always run a bit hot, if idling on a hot day the fan will turn on and the temp will rise a bit. The only time it's gotten into any danger zones was after a spirited 10 mile hill climb, and idling in the parking lot at the top.

Driving home, around town the rest of the day, etc. had no problem.

It was almost as if the fan wasn't turning on when the test was going on, because the car thought it was moving and didn't need the fan? Is this a "feature"?

Any help would be appreciated...

 

No, its not a feature. Your car should never heat up, tho. Even after a spirited run. You have air seeping into your cooling system somewhere, or air trapped in the system already.

 

Is the only fix for that a coolant flush?

 

Haha, no, you've gotta perform a pressure test and if there's a leak you gotta find it. If no leak, then you have to bleed the system of air.

If you don't know how to do this, go get a flush and ask them to pressure test the system. If there's no leak, you'll be fine with just the flush.