An Intoxilyzer 8000 and 8000C exception message that may occur during an air blank. If the air pump cannot bring the the instrument's perception of concentration in the sample chamber down to within 10 mg/100mls of the previous air blank, the instrument, after running the pump for awhile, will indicate "Purge fail". Purge fail is not limited to air blanks following the subject or simulator test. The phenomenon can also occur on an Intoxilyzer 5000C, but the phenomenon does not result in a specific error message. The cause of the phenomenon is floating zero on an IR instrument. Zero can be elevated for a number of reasons having to do with ambient conditions and operator error. Strangely, an "ambient fail" can be produced when an adverse ambient condition IS REMOVED. Note that zero can also be set below real zero on an Intoxilyzer 5000C as a result of radio frequency interference and cause a purge fail.