Ford Transit 2002 Owner's Manual page 1125

under full load.
A mechanism above the vacuum piston immobilises it in this position.
If the permeability of the air cleaner element is reduced even further, then the vacuum
increases again and the vacuum piston is pulled further down again.
The display on the indicator is divided into three areas with the following significance:
Green zone - air cleaner element is OK.
Clear zone - air cleaner element should be replaced.
Red zone - air cleaner element must be replaced.
NOTE: Under the effects of extreme humidity it is possible for the air cleaner condition
indicator to give an incorrect reading. If the air cleaner element has only recently been
replaced, but the vacuum piston is in the clear zone or the red zone, then press the reset
button. The air cleaner element does not need to be replaced.
After replacing the air cleaner element, the vacuum piston must be released by pressing
the reset button, which returns it to its starting position.
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