Carbon monoxide (CO) detectors can be used as a backup but not as a replacement for proper use and maintenance of you fuel-burning appliances.  CO detectors are not generally considered to be as reliable as smoke detectors. You should not choose a CO detector solely on the basis of cost;  do some research on different features available.
Carbon monoxide detectors should meet Underwriters Laboratories Inc. standards, have long-term warranty, and be easily self tested and reset to ensure proper functioning.  For maximum effectiveness during sleeping hours, carbon monoxide detectors should be placed close to sleeping areas.
If Your CO Detector Goes Off:
Make sure it is the CO detector and not smoke alarm
Check to see if any member of your household is experiencing symptoms.
If they are, get them out of the house immediately and seek medical attention
If no one is feeling symptoms, ventilate the home with fresh air and turn off all potential sources of CO.
Have a qualified technician inspect your appliances and chimneys to make sure they are operating correctly.