What indication did you have that the CO levels were high? Are you using a CO detector? Insuring that there is plenty of oxygen available to the burner will help reduce the CO. Yes, a blue flame generally indicates a cleaner burn and it's a good idea to keep the garage door open. The fan will also help insure a good supply of fresh air. I would not use propane indoors without a good CO detector. I like the digital ones that have an LED readout. It will warn you long before dangerous levels of CO are reached, but to take advantage of this you need to glance at it occasionally and see if it is registering a reading. IIRC, the alarm will trigger at 200 ppm CO, but it will also register much lower CO levels on the read out. OTOH, these units can fail I suppose, so it's prudent to ensure that you have adequate ventilation. Your life may depend on it.