This is an OCR of part of the manual.

OBDII Overview
        OBDII is the results of legislation enacted to keep vehide 
emission levels low. The two main pieces of legislation are the 
EPA's Federal Clean Air Act and requirements from the Califomia Air 
Resources Board, or CARB. The legislation charged auto makers with 
not only reducing vehicle emissions, but with monitonng the 
emission-related components and systems, as well. The OBDII 
legislation has the following selected requirements. The ECU must 
monitor the efficiency of the catalyst or heated catalyst, if 
equipped: the engine for misfire, the EVAP system, or leak detection 
pump: the secondary air system, the fuel system for nchllean 
conditions, all upstream 02 sensor sw~tch rates and heaters, the EGR 
system, and all comprehensive components" which can effect the 
emissions.

OBDII Monitors You have read about the different systems the ECU is 
required to monitor. Lets now talk about the two general types of 
OBDII monitors: continuous monitors and once-per-trip monitors. 
Continuous monitors run continuously dunng vehicle operation, as 
long as certain conditions are present. These are known as enabling 
conditions. An example is the 02 monitor w~II not run if the engine 
is cold. An enabling condition could be engine temperature greater 
than 140 F.

        There are two different continuous monitors: the misfire 
monitor and the fuel system richilean monitor. These two monitors 
have the highest pnonty. The misfire monitor uses information from 
the crankshaft position sensor to determine the cylinder to cylinder 
deceleration and infer a misfire. The misfire monitor will tum on 
the MIL in two different cases. First, if the misfire monitor 
detects engine misfire in approximately two percent of engine rpm in 
a 1,000 revolution penod, a temporary fault is set. These temporary 
faults are known as "pending faults". The ECU also records freeze 
frame data when the pending fault is detected. It takes two monitor 
failures in a row for the misfire monitor to tum on the MIL, for 
emissions increase misfire. If the vehicle fails the misfire monitor 
on the next running of the monitor, the pending fault matures, the 
MIL is tumed on, and a DTC is store. In the other case, if the 
misfire monitor detects engine misfire in more than 15 percent of 
cylinder finngs dunng any 200 revolutions, the MIL will begin 
flashing and a DTC is set immediately, along with freeze frame data. 
A flashing MIL indicates catalyst damage is about to occur.

        Let's look at the other OBDII continuous monitor - the fuel 
system nchllean monitor. The fuel system nchllean monitor looks at 
short and long term adaptive memory values to determine if the fuel 
system is in proper control. Once the enabling conditions are met, 
the fuel system is continuously monitored each tnp. Since the fuel 
system monitor is a two tnp monitor, a pending fault and freeze 
frame data are stored on the first failure. The MIL is not on at 
this point. But if the vehicle fails the fuel system monitor on the 
next tnp, a DTC is stored and the MIL is illuminated.

        Those are the OBDII continuous monitors. Let's look at the 
other types of monitors. The other type of OBDII monitors are known 
as once-per-trip monitors. They check components and systems that 
control emission levels for proper operation. Onc~per-tnp monitors 
run one time dunng a tnp and record the results as "Done (with no 
fault)" or Fail, in which case freeze frame data is stored. Vehi~es 
must meet the enabling conditions before a on~per-tnp monitor w~ll 
run. If the vehicte fails any of the on~per-tnp monitors on two 
successive trips, a DTC is recorded and the MIL is illuminated. The 
onc~per-tnp monitors are as follows:

•       EGR system Monitor
•       Oxygen sensor heater monitor
•       Oxygen sensor monitor
•       NC monitor if the vehicle uses CFC-12
•       Secondary Air system monitor
•       Evaporative system monitor
•       Heated catalyst monitor
•       Catalyst monitor

Like the continuous monitors, all of these once-per-tnp monitors 
have their own enabling conditions. The number of monitors used 
vanes from vehicle to vehide. For more detailed information on these 
monitors and other OBDII operation consult the factory service 
manuals.

OBDII Good-Trip Counters You've read about some of the ways the ECU 
tums the MIL on. Now, let's find out how it tums the MIL off. The 
ECU has software contained in its memory called good-trip counters. 
The sole purpose of these good-trip counters is to tum the MIL off 
after a repair has been made. The logic behind good-tnp counters is 
that if a fault that was previously set does not appear in three 
tnps in a row, the ECU requests the MIL to be tumed off. This is the 
OBDII way of letting the dnver know that the original problem is 
gone and the vehicle's emission levels are within acceptable limits. 
Just like the OBDII monitors, thereare different types of good-tnp 
counters.

        The first type is the global good-trip counter. The most 
common version of a global good tnp is to rerun the onc~per-tnp 
monitor that o'\ginally set the fault. If the vehicle passes the 
monitor, the global good-trip counter records one good tnp. 
Remember, the global good-tnp counter must see three good tnps in a 
row, with no failures, in order to tum the MIL off. The second type 
is the fuel system good4rip counter. The fuel system good-tnp 
counter is used to clear faults that were set by the fuel system 
monitor. It must count to three to tum off the MIL. The fuel system 
good tnp counter checks the fuel system adaptive memory for nchllean 
conditions, but only when the vehicle is under the same operating 
conditions as when the fault was onginallyrecorded. These conditions 
can be seen in the freeze frame screen. OBDII defines 
similar-conditions window as vehicle operation within plus or minus 
375 rpm and load conditions within 10 percent.

        The third type is the misfire good-trip counter. The misfire 
good-tnp counter only operates when the vehicle is in the similar 
conditions window. It tums off the MIL, or clears freeze frame data 
that was set by the misfire monitor. The Misfire good-tnp counter 
records one good tnp each time it sees 1,000 revolutions of the 
engine without a misfire, when in the similar-conditions window. It 
takes three good tnps to turn off the MIL.

OBDII Impact on Service The most obvious change is how faults are 
set. Prior to 1995, all faults were one-trip faults. If the ECU 
detected a problem, the MIL would come on right away. But, as you've 
seen, OBDII monitors are two-trip faults, except for the catalyst 
damage misfire monitor. Another change is that with OBDII, many more 
faults turn on the MIL. One reason for this increase in MIL- related 
codes is that the ECU must monitor system and some component 
deterioration, as well as its function, and can detect, for example, 
a sluggish 02 sensor.

        Just as OBDII caused changes in tuming on the MIL, it also 
changed the way the MIL is tumed off. Ea~ier systems, the MIL was 
tumed off by erasing the fault code memory in the ECU. But as you've 
seen, the MIL must stay on for at least three good tnps on OBDII 
vehicles.

        The many changes brought on by OBDII also changed the 
diagnostic philosophy. Before OBDII, it was standard practice to 
erase fault codes before a repair was made. Now, however, you should 
not clear fault codes from the ECU's memory until after the vehicle 
has run and passed either the monitor that set the fault, or the 
monitors required to increment the global good- trip counter.

 
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