Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
89
troubleshoot hard codes, figure 5-4. The procedures as-
sume that the problem is present at the time of testing.
Intermittent Codes
The term intermittent code indicates an intermittent
problem: one that comes and goes. The PCM records
most intermittent codes when they occur, then main-
tains a record of the fault in memory. An intermittent
code represents a problem that occurred some time in
the past, before testing, but is not present now. The
problem, and the DTC, may not reappear if PCM mem-
ory is cleared and the system retested. The conditions
to set the code may happen only at a certain speed,
temperature, or other condition that cannot be recreat-
ed in the shop.
Because intermittent codes indicate sporadic problems,
diagnostic charts usually do not isolate the problem im-
mediately. Generally, special intermittent, or symptom
diagnostic test procedures must be followed to trou-
bleshoot intermittent codes accurately. Do not open or
disconnect electrical connectors when intermittent
codes are present. This may temporarily solve a prob-
lem without revealing the basic cause.
Checking codes at this point is a preliminary step; it
does not provide a definitive answer as to the source of
the problem. Write down any codes in memory now for
future reference. Then start and run the engine, if possi-
ble, to verify that the system is operational and the PCM
is controlling engine functions.
Separating Hard and Intermittent Codes
When diagnosing pre-OBD II vehicles first determine
whether codes are hard or intermittent. Some systems
specify the type of code on the scan tool display, but
most do not. To distinguish between a hard and an in-
termittent code, record all codes, and then clear the
PCM memory. Codes can be cleared from all OBD II,
and most late-model vehicles using a scan tool.
On pre-OBD II vehicles codes can be cleared by mo-
mentarily disconnecting the battery or removing the fuse
for the PCM power circuit. OBD II systems store the
DTC's in a memory known as EEPROM and will not
erase when power is removed. In either case, refer to
the Service or Diagnostic Manual for the correct proce-
dure.
After clearing the codes, drive the vehicle and watch for
the MIL to light, an indication that the code has reset. If
a code reappears immediately or soon, a hard fault is in-
dicated. If the code does not reappear quickly, it was
most likely an intermittent code. To diagnose any DTC,
go to the test or troubleshooting chart for that particu-
lar code.
Pre-OBD II vehicles trouble codes should be diagnosed
and serviced in a basic order: hard codes first, followed
by intermittent codes. Most of those vehicles transmit
codes in numerical order from the lowest to the highest,
and this is usually the order in which they should be ser-
viced. Exceptions would be codes that indicate com-
munications and internal PCM problems. These are pri-
ority situations that should be serviced first, before any
other codes.
OBD II vehicles generate codes based on a much more
complex basis. Start your diagnosis of multiple code sit-
uations by diagnosing the first code in memory. If other
DTC's should be serviced first, the Service or Diagnos-
tic Manual will direct you to do so.
OBD II System Operation
Comprehensive Component Monitor
The OBD II diagnostic system continuously monitors all
engine and transmission sensors and actuators for
shorts, opens, and out-of-range values, as well as val-
ues that do not logically fit with other powertrain data
(rationality). On the first trip during which the
compre-
hensive
component monitor detects a failure that will
result in emissions exceeding a predetermined level, the
ECM will store a DTC, illuminate the MIL, and store a
freeze
frame.
System Monitors
The OBD II diagnostic system also actively tests some
systems for proper operation. While the vehicle is being
driven fuel control and engine misfire are checked con-
tinuously. Oxygen sensor response, oxygen sensor
heater operation, catalyst efficiency, EGR operation,
EVAP integrity, variable valve timing, and thermostat op-
eration are tested once or more per trip. When any of the
System Monitors detects a failure that will result in emis-
sions exceeding a predetermined level on two consec-
utive trips, the ECM will store a diagnostic trouble code
(DTC) and illuminate the malfunction indicator lamp
(MIL). Freeze frame data captured during the first of the
two consecutive failures is also stored.
FUEL CONTROL - For many vehicles the monitor will
set a DTC if the system fails to enter Closed Loop mode
within few minutes of startup, or the Long Term Fuel
Trim is excessively high or low anytime after the engine
is warmed up, indicating the loss of fuel control. This is
always the case when the Long Term Fuel Trim reaches
its limits.
ENGINE MISFIRE - For many vehicles, the monitor uses
the CKP sensor signal to continuously detect engine
misfires both severe and non-severe. If the misfire is se-
vere enough to cause catalytic converter damage, the
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