Steve,
I believe also it is in the ECU...Fuel injection has a cold start circuit. That is controlled through the ECM of whatever vehicle it is.
Cold start stumbles, typically are a result of misinformation being sent (sensor) to the ECM. Hence the ECM will produce a lean condition.

Sensors are hard to diag. as they have gremlins and we all know how hard they are to catch...

Has anyone had a code thrown due to the stumble??