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fuel trim refers to dynamic or instantaneous adjustments. Long-term fuel trim
refers to much more gradual adjustments to the fuel calibration schedule than
short-term trim adjustments. These long-term adjustments compensate for
vehicle dierences and gradual changes that occur over time.
2.6 OBD II Monitors
An important part of a vehicle’s OBD II system is the Readiness Monitors,
which are indicators used to nd out if all of the emissions components have
been evaluated by the OBD II system. They are running periodic tests on
specic systems and components to ensure that they are performing within
allowable limits.
Monitor operation is either “Continuous” or “Non-Continuous,” depending on
the specic monitor.
2.6.1 Continuous Monitors
Some of the vehicle components or systems are continuously tested by the
vehicle’s OBD II system, while others are tested only under specic vehicle
operating conditions. The continuously monitored components listed below are
always ready:
1. Misre Monitor
This Monitor continuously checks for engine misres. A misre occurs when
the air-fuel mixture in the cylinder does not ignite. The misre Monitor uses
changes in crankshaft speed to sense an engine misfire. When a cylinder
misres, it no longer contributes to the speed of the engine, and engine speed
decreases each time the affected cylinder(s) misfire. The misfire Monitor
is designed to sense engine speed fluctuations and determine from which
cylinder(s) the misre is coming, as well as how bad the misre is.
There are three types of engine misres, Types 1, 2, and 3.
• Type 1 and Type 3 misres are two-trip monitor faults. If a fault is sensed
on the rst trip, the computer temporarily saves the fault in its memory as
a Pending Code. The MIL is not commanded on at this time. If the fault is
found again on the second trip, under similar conditions of engine speed,
load and temperature, the computer commands the MIL “On,” and the code
is saved in its long term memory.
• Type 2 misres are the most severe type of misre. When a Type 2 misre
is sensed on the rst trip, the computer commands the MIL to light when
the misre is sensed. If the computer determines that a Type 2 misre is
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