CRP909E User Manual

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Professional 909E User Manual

2.6 OBD II Monitors

An important part of a vehicle’s OBD II system is the Readiness Monitors, which

are indicators used to find out if all of the emissions components have been

evaluated by the OBD II system. They are running periodic tests on specific

systems and components to ensure that they are performing within allowable

limits.

Monitor operation is either “Continuous” or “Non-Continuous,” depending on the

specic 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 specific vehicle

operating conditions. The continuously monitored components listed below are

always ready:

1. Misre Monitor

This Monitor continuously checks for engine misres. A misre occurs when the

air-fuel mixture in the cylinder does not ignite. The misre Monitor uses changes

in crankshaft speed to sense an engine misre. When a cylinder misres, it no

longer contributes to the speed of the engine, and engine speed decreases each

time the affected cylinder(s) misre. The misre Monitor is designed to sense

engine speed fluctuations and determine from which cylinder(s) the misfire is

coming, as well as how bad the misre is.

There are three types of engine misres, Types 1, 2, and 3.

• Type 1 and Type 3 misfires 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 misres are the most severe type of misre. When a Type 2 misre

is sensed on the rst trip, the computer commands the MIL to light when the

misre is sensed. If the computer determines that a Type 2 misre is severe,

and may cause catalytic converter damage, it commands the MIL to “ash”

once per second as soon as the misfire is sensed. When the misfire is no

longer present, the MIL reverts to steady “On” condition.

The Misfire Monitor is supported by both “spark ignition” vehicles and

“compression ignition” vehicles.

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User Manual - Page 15