Monitor Operation
To monitor catalyst efficiency, PCM expands rich and lean switch points of heated oxygen sensor. With extended switch points, air/fuel mixture runs richer and leaner to overburden catalytic converter. Once test is started, air/fuel mixture runs rich and lean and O2 switches are counted. A switch is counted when an oxygen sensor signal goes from below lean threshold to above rich threshold. The number of rear O2 sensor switches is divided by number of front O2 sensor switches to determine switching ratio.
The test runs for 20 seconds. As catalyst efficiency deteriorated over life of vehicle, switch rate at downstream sensor approaches that of upstream sensor. If at any point during test period switch ratio reaches a predetermined value, a counter is incremented by one. The monitor is enabled to run another test during that trip. When test fails 7 times, counter increments to 7, a malfunction is entered, and a Freeze Frame is stored, code is matured and MIL is illuminated. If test passes first, no further testing is conducted during that trip.
The MIL is extinguished after 3 consecutive good trips. The good trip criteria for catalyst monitor is more stringent than failure criteria. In order to pass test and increment one good trip, downstream sensor switch rate must be less than 65% of upstream rate. The failure percentages are 75% respectively.
Enabling Conditions - The following conditions must typically be met before PCM runs catalyst 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
- 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 (Auto Bans Only)
- Intake Air Temperature
Conflict - The catalyst monitor does not run if any of the following conditions are present:
- EGR Monitor In Progress (if equipped)
- Fuel System Rich Intrusive Test In Progress
- EVAP Monitor In Progress
- Time Since Start Is Less Than 60 Seconds
- Low Fuel Level Less Than 15%
- Low Ambient Air Temperature
- Ethanol Content Learn Is Taking Place And The Ethanol Used Once Flag Is Set
Suspend - The Task Manager does not mature a catalyst fault if any of the following are present:
- Oxygen Sensor Monitor, Priority 1
- Oxygen Sensor Heater, Priority 1
- EGR Monitor, Priority 1 (if equipped)
- EVAP Monitor, Priority 1
- Fuel System Monitor, Priority 2
- Misfire Monitor, Priority 2