Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
90
MIL will blink as long the severe misfire is detected.
CATALYTIC CONVERTER - For many vehicles the mon-
itor compares the signals of the upstream heated oxy-
gen sensors to the signal from the downstream heated
oxygen to determine the ability of the catalyst to store
free oxygen. If the converter's oxygen storage capacity
is sufficiently degraded, a DTC is set.
EGR SYSTEM - For many vehicles the monitor uses the
MAP sensor signal to detect changes in intake manifold
pressure as the EGR valve is commanded to open and
close. If the pressure changes too little or too much, a
DTC is set.
EVAP SYSTEM - For many vehicles the monitor first
turns on the EVAP vent solenoid to block the fresh air
supply to the EVAP canister. Next, the EVAP purge so-
lenoid is turned on to draw a slight vacuum on the en-
tire EVAP system, including the fuel tank. Then the EVAP
purge solenoid is turned off to seal the system. The
monitor uses the fuel tank (EVAP) pressure sensor sig-
nal to determine if the EVAP system has any leaks. If the
vacuum decays too rapidly, a DTC is set. In order to run
this monitor, the engine must be cold and the fuel level
must be between 1/4 and 3/4 full.
VARIABLE VALVE TIMING - For many vehicles the mon-
itor compares the desired valve timing with the actual
timing indicated by the CMP sensors. If the timing is in
error, or takes too long to reach the desired value, a DTC
is set.
ENGINE THERMOSTAT - This monitor confirms that the
engine warms up fully within a reasonable amount of
time. If the coolant temperature remains too low for too
long, a DTC is set.
OXYGEN SENSORS - This monitor checks the maxi-
mum and minimum output voltage, as well as switching
and response times for all oxygen sensors. If an oxygen
sensor signal remains too low or too high, switches too
slowly, or not at all, a DTC is set.
OXYGEN SENSOR HEATERS - This monitor checks the
time from cold start until the oxygen sensors begin to
operate. If the time is too long, a DTC is set. Battery volt-
age is continuously supplied to the oxygen sensor
heaters whenever the ignition switch is on.
Comprehensive Component Monitors
Comprehensive component monitors are most like the
OBD I monitoring system that watches engine and
transmission sensor inputs and actuator outputs for
shorts, opens and out-of-range values. OBD II comput-
er (ECM) programs are enhanced to include identifica-
tion of sensor values that don't logically fit with other
powertrain data. For instance, if the Throttle Position
Sensor (TPS) is reporting wide-open, but other sensors
are reporting idle speed values, the ECM will set a DTC
for the TPS.
Remember, comprehensive component monitors are
one trip monitors. The ECM will activate the MIL and
store DTC and freeze frame data the first time an emis-
sions related fault is detected. If a misfire or fuel control
problem is detected after the original DTC was record-
ed, freeze frame date for the misfire or fuel control code
will replace the original data.
Readiness Status and System Monitors
You will recall that the monitor readiness status tells the
technician if a particular diagnostic monitor (test) has
been completed since the last time DTCs were cleared
from memory. There are two important concepts to un-
derstand when viewing monitor readiness status: First,
the vehicle must be driven under specific conditions for
some monitors to run, and second, the emissions sys-
tem being monitored must be operational. If battery
power is disconnected and the vehicle isn't driven
through an entire drive cycle, the readiness status will
be "NO". If there is an electrical problem or component
failure in a monitored system, the monitor will not run. A
DTC may be recorded that points to the electrical or
component failure, but the system cannot be tested by
the monitor, so the readiness status will be "NO". A
readiness status of "NO" for any of the five monitored
systems, catalyst, EGR, EVAP, Oxygen sensors, and
Oxygen sensor heaters does not mean a failed monitor,
only that the monitor has not been completed. At the
same time, a "YES" status does not mean the system
passed the monitor, only that the test was completed. In
both cases, you must check for codes to investigate fur-
ther.
Fuel Control Monitor
The fuel control monitor is designed to constantly check
the ability of the ECM to control the air/fuel ratio. For
many vehicles the ECM program that fine tunes the
air/fuel ratio is called Fuel Trim. It is divided into a short
term program and a long term program. Both trim pro-
grams are presented as diagnostic data when a freeze
frame is recorded. Separate short term and long term
data is displayed for cylinder bank 1 and cylinder bank
2.
The oxygen sensor (HO2S) drives the fuel trim program
anytime the vehicle is in closed loop. The starting point
for fuel trim is 0% correction, figure 5-5. When the ECM
sees a lean (low voltage) signal from an upstream HO2S,
the fuel trim program adds fuel to compensate for the
detected leanness. The short term fuel trim display on
the scan tool will move to the positive (+) side of 0% to
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