I have a question about CO symptoms. I'm going to get a detector,but if there is a leak in the furnace, even a low level, wouldn't my dogs and cat show symptoms like headache, fatigue, etc? :confused:

As far as I know, carbonmonoxide exposure will cause (at low levels) general illness,
nausea, headache, tired. But CO is extrmemly dangerous, as you have no way of knowing it is present until it is too late. IF you suspect a CO problem, please make sure the house is well ventilated and get a detector as soon as possible, or have your furnace serviced asap. Please don't wait until it is too late, as your pets will be affected sooner than you, due to their smaller size.
And they can't tell you when they feel sick.