O

O - Chemical symbol for oxygen.
ON-DEMAND TEST - In self-test, the "key on/engine off" and "engine running" positions, which are tectnician initiated, are run from a program within the ECA.

OPEN LOOP MODE - Mode in which the ECA operates without EGO sensor feedback.

-Open Loop Switch:

OPEN LOOP SWITCH - A normally opened or normally closed vacuum operated electric switch.
OPEN SYSTEM - Descriptive term for crankcase emissions control system which vents to the atmosphere.

OUTPUT CYCLING CHECK (OCC) - A position of the end of the "key on/engine off" test that enables energizing and de-energizing the auxiliary output on demand.

OUTPUT STATE CHECK - A test performed in the "key on/engine off" mode after the continuous codes have been sent which forces the processor to activate some actuators for additional diagnostics.

OVERBOOST WARNING SWITCH - Switch assembly which has a pressure switch that causes a light or buzzer to come on when turbocharger boost pressure is excessive.