WARNING Fresh air/zero calibrations may only be performed in an atmosphere that is known to contain 20.9% oxygen, 0% LEL, and 0 ppm toxic gas. 1. Initiate the standard fresh air/zero calibration sequence by pressing the MODE button three times in rapid succession. The second countdown will begin. Press the MODE button before the end of the second countdown and continue to hold the MODE button. As in the standard fresh air/ zero calibration, the IQ Force will show cascading zeros. The fresh air/zero calibration is complete when the instrument begins another second countdown for the span calibration. If span calibration is not required, allow the countdown to reach 0 without pressing the MODE button. If the IQ Force still fails to calibrate after attempting to force the fresh air/ zero calibration, contact the local service center.