Temp sensor problem?
 

2005 Boxster S. A few minutes after startup, the temp is still pegged at cold. The coolant temp warning light comes on. Minutes later the temp rises to normal and the light goes out. Is this a sensor issue or maybe a thermostat stuck partly open?

Did you check coolant level?

Sensor problem
 

Yes, coolant level is just above minimum.

I’d suggest that getting it up to maximum would be your first step.