the number of Front O2 sensor switches to determine
the switching ratio.
The test runs for 20 seconds. As catalyst efficiency
deteriorated over the life of the vehicle, the switch
rate at the downstream sensor approaches that of the
upstream sensor. If at any point during the test
period the switch ratio reaches a predetermined
value, a counter is incremented by one. The monitor
is enabled to run another test during that trip. When
the test fails three times, the counter increments to
three, a malfunction is entered, and a Freeze Frame
is stored. When the counter increments to three dur-
ing the next trip, the code is matured and the MIL is
illuminated. If the test passes the first, no further
testing is conducted during that trip.
The MIL is extinguished after three consecutive
good trips. The good trip criteria for the catalyst
monitor is more stringent than the failure criteria. In
order to pass the test and increment one good trip,
the downstream sensor switch rate must be less than
80% of the upstream rate (60% for manual transmis-
sions). The failure percentages are 90% and 70%
respectively.
Enabling Conditions— The following conditions
must typically be met before the PCM runs the cat-
alyst monitor. Specific times for each parameter may
be different from engine to engine.
• Accumulated drive time
• Enable time
• Ambient air temperature
• Barometric pressure
• Catalyst warm-up counter
• Engine coolant temperature
• Accumulated throttle position sensor
• Vehicle speed
• MAP
• RPM
• Engine in closed loop
• Fuel level
Pending Conditions—
• Misfire DTC
• Front Oxygen Sensor Response
• Front Oxygen Sensor Heater Monitor
• Front Oxygen Sensor Electrical
• Rear Oxygen Sensor Rationality (middle check)
• Rear Oxygen Sensor Heater Monitor
• Rear Oxygen Sensor Electrical
• Fuel System Monitor
• All TPS faults
• All MAP faults
• All ECT sensor faults
• Purge flow solenoid functionality
• Purge flow solenoid electrical
• All PCM self test faults
• All CMP and CKP sensor faults
• All injector and ignition electrical faults
• Idle Air Control (IAC) motor functionality
• Vehicle Speed Sensor
• Brake switch
• Intake air temperature
Conflict— The catalyst monitor does not run if
any of the following are conditions are present:
• EGR Monitor in progress
• Fuel system rich intrusive test in progress
• EVAP Monitor in progress
• Time since start is less than 60 seconds
• Low fuel level
• Low ambient air temperature
Suspend— The Task Manager does not mature a
catalyst fault if any of the following are present:
• Oxygen Sensor Monitor, Priority 1
• Upstream Oxygen Sensor Heater, Priority 1
• EGR Monitor, Priority 1
• EVAP Monitor, Priority 1
• Fuel System Monitor, Priority 2
• Misfire Monitor, Priority 2
NON-MONITORED CIRCUITS
OPERATION
The PCM does not monitor all circuits, systems
and conditions that could have malfunctions causing
driveability problems. However, problems with these
systems may cause the PCM to store diagnostic trou-
ble codes for other systems or components. For exam-
ple, a fuel pressure problem will not register a fault
directly, but could cause a rich/lean condition or mis-
fire. This could cause the PCM to store an oxygen
sensor or misfire diagnostic trouble code.
The major non-monitored circuits are listed below
along with examples of failures modes that do not
directly cause the PCM to set a DTC, but for a sys-
tem that is monitored.
FUEL PRESSURE
The fuel pressure regulator controls fuel system
pressure. The PCM cannot detect a clogged fuel
pump inlet filter, clogged in-line fuel filter, or a
pinched fuel supply or return line. However, these
could result in a rich or lean condition causing the
PCM to store an oxygen sensor or fuel system diag-
nostic trouble code.
SECONDARY IGNITION CIRCUIT
The PCM cannot detect an inoperative ignition coil,
fouled or worn spark plugs, ignition cross firing, or
open spark plug cables.
CYLINDER COMPRESSION
The PCM cannot detect uneven, low, or high engine
cylinder compression.
25 - 18 EMISSION CONTROL SYSTEMS LH
DESCRIPTION AND OPERATION (Continued)
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