System Outline

WARNING: This page is about a different car, the 2011 Honda Insight and 2010 Honda Insight. However, it is still accessible from the selected car via links, so may be relevant.

During idle stop or when all cylinders are deactivated, vacuum inside the intake manifold lowers.

Vacuum for the brake booster is monitored by PCM. Idle stop and cylinder deactivation takes place as long as there is sufficient vacuum.

When the brake booster vacuum is less than the reference value, cylinder deactivation and idle stop will not occur.
Cylinder deactivation and idle stop are monitored constantly and stopped by the PCM if it detects a failure. (Two sensors monitor mutually characteristic abnormalities.)
The IMA indicator comes on when brake booster pressure sensor(s) fail.

Fig 1: Identifying Intake Manifold, Check Valve And Brake Booster
Courtesy of AMERICAN HONDA MOTOR CO., INC.

The brake booster vacuum must reach the established point to ensure enough braking force to allow for auto idle stop activation. If the established value is decreased to the extent that braking force cannot be ensured, the engine will transition from auto idle stop to the engine idling.

Fig 2: Identifying Vacuum Pressure Of Brake Booster And Auto Idle Stop Conditions

Courtesy of AMERICAN HONDA MOTOR CO., INC.