Toyota Celica, Corolla, ECHO & MR2 1999-05

OBD II Monitors


Catalyst Monitor

A downstream (post-catalyst) Heated Oxygen Sensor (HO2S-12) is used to provide the additional signals needed to monitor the efficiency of the three-way catalyst on these systems. The PCM compares the signals between the upstream (pre-catalyst) and downstream oxygen sensor during stable driving conditions with the engine warm in order to determine the oxygen storage capacity of the catalytic converter.

Catalyst Monitor "Trip" Pattern

The Catalyst Monitor "trip" pattern shown below can be used to validate repair of DTC P0420/P0430 or to "run" the Catalyst Monitor to complete the I/M Readiness Test. Federal models require that the vehicle be driven at a different speed than for California models. If the engine is cold on 2002 models, repeat the test (key off after the test).

Click image to see an enlarged view


Catalyst Monitor Operation

To measure catalyst efficiency, the Catalyst Monitor interprets the signals from the pre-catalyst and post-catalyst oxygen sensors. If the three-way catalyst is operating correctly, the post-catalyst signal will have significantly less activity than the pre-catalyst. Run the engine at 2500 rpm for 3 minutes in P/N and then check the waveforms from both sensors. If the signals are similar, the catalyst may be degraded.

Comprehensive Component Monitor

The Comprehensive Component Monitor (CCM) is an on-board strategy designed to monitor for failures in emission-related electronic components and circuits that provide input or output signals to the PCM. These are systems or devices that are not exclusively monitored by another monitor system. If the PCM detects that an input or output signal is inoperative due to an out-of-range value, open circuit or if an on-board rationality or functionality check fails, the PCM will set a code in memory and activate the MIL.

Tests conducted by the CCM vary depending on the type of hardware, the function of the device and the signal type. Analog signals are checked continuously for opens, shorts and out-of-range values. Some digital signals are checked for both functionality and rationality. These tests require that certain engine conditions be present before the test is performed and that several components are monitored as part of the test. Also, a sensor value can be monitored for change after the PCM sends a command to a device. Here is a list of devices checked by the CCM at key on or engine running.

Input Device Examples

Barometric Pressure Sensor
Brake Switch
Camshaft & Crankshaft Sensors
Clutch Switch (M/T)
Cruise Servo Switch
Engine Coolant Temperature Sensor
EVAP Pressure Sensor
Intake Air Temperature Sensor
Knock Sensor
Manifold Absolute Pressure Sensor
Mass Airflow Sensor
Park Neutral Switch
Transmission Temperature Sensor
Transmission Speed Sensor
Vehicle Speed Sensor

Output Device Examples

EVAP Purge and Vent Solenoids
Idle Air Control Solenoid
Ignition Control System
Transmission Torque Converter Clutch Solenoid
Transmission Shift Solenoids (Solenoid S1, S2 and S3)

EGR System Monitor

The EGR system is used to reduce the amount of NOx emissions by circulating a portion of the exhaust gas through the EGR valve to the intake manifold. The PCM uses the MAP sensor signal to detect an EGR system fault by determining if the signal is too high or too low.

Click image to see an enlarged view


EGR System Monitor "Trip" Pattern

The "trip" pattern below can be used to validate repair of DTC P0401 or to "run" the EGR Monitor to complete the I/M Readiness Test.

Click image to see an enlarged view


EGR Vacuum Modulator

An EGR vacuum modulator that operates according to engine load controls the amount of EGR gas flow. The PCM turns on the vacuum modulator to allow atmospheric air to "act" on the EGR valve, and to shut off the flow of EGR gas to the intake manifold with a cold engine, during high or low engine load, at high speed, deceleration or at idle.

Main Monitors (OBD II Monitors)

A key difference between the first version of On Board Diagnostics (OBD I) and the second version (OBD II) is the use of several PCM controlled monitors contained within the PCM software structure. These monitors perform diagnostic tests required in order to meet specific California Air Resources Board (CARB) and EPA regulations.

Simply stated, an OBD II Monitor is a diagnostic strategy designed to test the operation of an emissions-related component or system. Some OBD II System Monitors accomplish this task directly by monitoring the action of various input and output devices or sensors connected to the PCM. An example of direct monitoring is when the Comprehensive Component Monitor monitors the Engine Coolant Temperature or Intake Air Temperature Sensor inputs.

Other OBD II System Monitors accomplish the task indirectly by monitoring the effects of changes to a system or component. The indirect method may be accomplished through monitoring a change or response in a system. This type of test is done by monitoring the input or output signals of a particular device for an "inferred" change.

An example of indirect monitoring is when the PCM infers correct or incorrect catalyst action using the Catalyst Monitor to sample signals from the upstream or downstream oxygen sensors. This allows the PCM to determine the oxygen storage efficiency of the catalyst.

Some of the Main Monitors run continuously while some run only once per trip. The next few articles explain how the Monitors operate.

Continuous Monitors (Run All The Time)

Fuel Control System - begins when the engine enters closed loop
Misfire Detection Test - begins right after startup

Main Monitors That Run Once Only Per Trip:

Catalyst Efficiency Test - begins in closed loop after certain engine temperature, time and VSS requirements are met
EGR System - begins in closed loop after certain engine temperature, time and VSS requirements are met
EVAP System Test - begins in closed loop after certain engine temperature, time and VSS requirements are met
O2S Test - voltage and response time tests begin in closed loop after engine temperature, time and VSS requirements are met
Secondary AIR System Test - begins in closed loop at off-idle

Note: Once all of the required enable criteria are met, Toyota OBD II systems are programmed to run all the OBD II main monitors once each trip.