“Zero Drift” refers to the amount of soot or build-up on your lens set. When the system first calibrates, the meter checks and references your “zero” . It then stores that value. After your testing, the meter then rechecks the “zero” value, and stores that value. It then compares both values and determines the meter is returning to “zero”. If it does not, that means the lenses have accumulated soot, and the test is invalid.

 
It is important that the sensor head be removed from the stack both with the initial calibration, and before taking your final “zero” check. Even with the engine off, hot gases are still present, and can affect your final reading.