I just had a water pump and belt tensioner replaced on tuesday, and had the coolant filled and the system pressure checked at that time. I drove to tahoe to spend the week here, and the check engine light came on. OBD2 codes were P0116 and P3999. Coolant looks fine and i haven't been running hot, so i assume its the thermostat or a sensor. I have reset the codes and they haven't come back. If you were me, would you get it checked out, or just keep an eye on the fluids until i head home next week? Also, i couldn't find P3999 anywhere, though some have stated on the forum that it may be a bad O2 sensor.