There are several components that will affect vehicle emissions if they malfunction. If one of these components malfunctions the Malfunction Indicator Lamp (Check Engine) will illuminate.

Some of the component monitors are checking for proper operation of the part. Electrically operated components now have input (rationality) and output (functionality) checks as well as continuity tests (opens/shorts). Previously, a component like the Throttle Position sensor (TPS) was checked by the PCM for an open or shorted circuit. If one of these conditions occurred, a DTC was set. Now there is a check to ensure that the component is working. This is done by watching for a TPS indication of a greater or lesser throttle opening than MAP and engine RPM indicate. In the case of the TPS, if engine vacuum is high and engine RPM is 1600 or greater and the TPS indicates a large throttle opening, a DTC will be set. The same applies to low vacuum and 1600 RPM.

Any component that has an associated limp in will set a fault after 1 trip with the malfunction present.

The following is a list of the monitored components:

• Catalyst Monitor
• Comprehensive Components
• EGR (if equipped)
• Fuel Control (rich/lean)
• Oxygen Sensor Monitor
• Oxygen Sensor Heater Monitor
• Purge
• Misfire
• ESIM (Evaporative System Integrity Monitor)

COMPREHENSIVE COMPONENTS

Along with the major monitors, OBD II requires that the diagnostic system monitor any component that could affect emissions levels. In many cases, these components were being tested under OBD I. The OBD I requirements focused mainly on testing emissions-related components for electrical opens and shorts.

However, OBD II also requires that inputs from powertrain components to the PCM be tested for rationality, and that outputs to powertrain components from the PCM be tested for functionality. Methods for monitoring the various Comprehensive Components include:

1. Circuit Continuity

• Open
• Shorted high
• Shorted to ground

2. Rationality or Proper Functioning

NOTE: Comprehensive component monitors are continuous. Therefore, enabling conditions do not apply. All will set a DTC and illuminate the MIL in 1- trip.

• Inputs tested for rationality
• Outputs tested for functionality

Input Rationality- While input signals to the PCM are constantly being monitored for electrical opens and shorts, they are also tested for rationality. This means that the input signal is compared against other inputs and information to see if it makes sense under the current conditions.

PCM sensor inputs that are checked for rationality include:

• Manifold Absolute Pressure (MAP) Sensor
• Oxygen Sensor (O2S) (slow response)
• Engine Coolant Temperature (ECT) Sensor
• Camshaft Position (CMP) Sensor
• Vehicle Speed Sensor
• Crankshaft Position (CKP) Sensor
• Inlet Air Temperature (IAT) Sensor
• Power Steering Switch
• Oxygen Sensor Heater
• Engine Controller
• Brake Switch
• ESIM (Evaporative System Integrity Monitor)
• P/N Switch
• Trans Controls

Output Functionality- PCM outputs are tested for functionality in addition to testing for opens and shorts.

When the PCM provides a voltage to an output component, it can verify that the command was carried out by monitoring specific input signals for expected changes. For example, when the PCM commands the Idle Air Control (IAC) Motor to a specific position under certain operating conditions, it expects to see a specific (target) idle speed (RPM). If it does not, it stores a DTC.

PCM outputs monitored for functionality include:

• Fuel Injectors
• Ignition Coils
• Torque Converter Clutch Solenoid
• Purge Solenoid
• EGR Solenoid
• Radiator Fan Control
• Trans Controls

OXYGEN SENSOR (O2S) MONITOR

DESCRIPTION- Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches operating temperature 300º to 350ºC (572º to 662ºF), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. When there is a large amount of oxygen in the exhaust caused by a lean condition, misfire or exhaust leak, the sensor produces a low voltage, below 450 mV. When the oxygen content is lower, caused by a rich condition, the sensor produces a higher voltage, above 450mV.

The information obtained by the sensor is used to calculate the fuel injector pulse width. The PCM is programmed to maintain the optimum air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NOx) from the exhaust.

The O2S is also the main sensing element for the EGR, Catalyst and Fuel Monitors, and purge.

The O2S may fail in any or all of the following manners:

• Slow response rate (Big Slope)
• Reduced output voltage (Half Cycle)
• Heater Performance

Slow Response Rate (Big Slope)- Response rate is the time required for the sensor to switch from lean to rich signal output once it is exposed to a richer than optimum A/F mixture or vice versa. As the PCM adjusts the air/fuel ratio, the sensor must be able to rapidly detect the change. As the sensor ages, it could take longer to detect the changes in the oxygen content of the exhaust gas. The rate of change that an oxygen sensor experiences is called 'Big Slope'. The PCM checks the oxygen sensor voltage in increments of a few milliseconds.

Reduced Output Voltage (Half Cycle)- The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value. Many times the condition is only temporary and the sensor will recover. Under normal conditions the voltage signal surpasses the threshold, and a counter is increments by one. This is called the Half Cycle Counter.

Heater Performance- The heater is tested by a separate monitor.

OPERATION- As the Oxygen Sensor signal switches, the PCM monitors the half cycle and big slope signals from the oxygen sensor. If during the test neither counter reaches a predetermined value, a malfunction is entered and a Freeze Frame is stored. Only one counter reaching its predetermined value is needed for the monitor to pass.

The Oxygen Sensor Signal Monitor is a two trip monitor that is tested only once per trip. When the Oxygen Sensor fails the test in two consecutive trips, the MIL is illuminated and a DTC is set. The MIL is extinguished when the Oxygen Sensor monitor passes in three consecutive trips. The DTC is erased from memory after 40 consecutive warm-up cycles without test failure.

Enabling Conditions- The following conditions must typically be met for the PCM to run the oxygen sensor monitor:

• Battery voltage
• Engine temperature
• Engine run time
• Engine run time at a predetermined speed
• Engine run time at a predetermined speed and throttle opening
• Transmission in gear (automatic only)
• Fuel system in Closed Loop
• Long Term Adaptive (within parameters)
• Power Steering Switch in low PSI (no load)
• Engine at idle
• Fuel level above 15%
• Ambient air temperature
• Barometric pressure
• Engine RPM within acceptable range of desired idle
• Closed throttle speed

Pending Conditions- The Task Manager typically does not run the Oxygen Sensor Signal Monitor if overlapping monitors are running or the MIL is illuminated for any of the following:

• Misfire Monitor
• Front Oxygen Sensor and Heater Monitor
• MAP Sensor
• Vehicle Speed Sensor
• Engine Coolant Temperature Sensor
• Engine Controller Self Test Faults
• Cam or Crank Sensor
• Injector and Coil
• EVAP Electrical
• EGR Solenoid Electrical
• Inlet Air Temperature
• 5 Volt Feed

Conflict- The Task Manager does not run the Oxygen Sensor Monitor if any of the following conditions are present:

• A/C ON (A/C clutch cycling temporarily suspends monitor)
• Purge flow in progress

Suspend- The Task Manager suspends maturing a fault for the Oxygen Sensor Monitor if an of the following are present:

• Oxygen Sensor Heater Monitor, Priority 1
• Misfire Monitor, Priority 2

OXYGEN SENSOR HEATER MONITOR (NGC)

DESCRIPTION- If the Oxygen sensor (O2S) DTC as well as a O2S heater DTC is present, the O2S Heater DTC MUST be repaired first. After the O2S Heater is repaired, verify that the sensor circuit is operating correctly.

The voltage reading taken from the O2S are very temperature sensitive. The readings taken from the O2S are not accurate below 300 degrees C. Heating the O2S is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2S must be tested to ensure that it is heating the sensor properly. Starting with the introduction on the NGC module the strategy for checking the heater circuit has changed. The heater resistance is checked by the NGC almost immediately after the engine is started.

The same O2S heater return pin used to read the heater resistance is capable of detecting an open circuit, a shorted high or shorted low condition.

CATALYST MONITOR

To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emission of hydrocarbons, oxides of nitrogen and carbon monoxide.

Normal vehicle miles or engine misfire can cause a catalyst to decay. A meltdown of the ceramic core can cause a reduction of the exhaust passage. This can increase vehicle emissions and deteriorate engine performance, driveability and fuel economy.

The catalyst monitor uses dual oxygen sensors (O2S's) to monitor the efficiency of the converter. The dual O2S strategy is based on the fact that as a catalyst deteriorates, its oxygen storage capacity and its efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2S is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter. The PCM calculates the A/F mixture from the output of the O2S. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).

When the upstream O2S detects a high oxygen condition, there is an abundance of oxygen in the exhaust gas. A functioning converter would store this oxygen so it can use it for the oxidation of HC and CO. As the converter absorbs the oxygen, there will be a lack of oxygen downstream of the converter. The output of the downstream O2S will indicate limited activity in this condition.

As the converter loses the ability to store oxygen, the condition can be detected from the behavior of the downstream O2S. When the efficiency drops, no chemical reaction takes place. This means the concentration of oxygen will be the same downstream as upstream. The output voltage of the downstream O2S copies the voltage of the upstream sensor. The only difference is a time lag (seen by the PCM) between the switching of the O2S's.

To monitor the system, the number of lean-to-rich switches of upstream and downstream O2S's is counted. The ratio of downstream switches to upstream switches is used to determine whether the catalyst is operating properly. An effective catalyst will have fewer downstream switches than it has upstream switches i.e., a ratio closer to zero. For a totally ineffective catalyst, this ratio will be one-to-one, indicating that no oxidation occurs in the device.

The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL (check engine lamp) will be illuminated.

Monitor Operation- To monitor catalyst efficiency, the PCM expands the rich and lean switch points of the heated oxygen sensor. With extended switch points, the air/fuel mixture runs richer and leaner to overburden the catalytic converter. Once the test is started, the air/fuel mixture runs rich and lean and the O2 switches are counted. A switch is counted when an oxygen sensor signal goes from below the lean threshold to above the rich threshold. The number of Rear O2 sensor switches is divided by 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 increments 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 during 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 transmissions). The failure percentages are 90% and 70% respectively.

Enabling Conditions- The following conditions must typically be met before the PCM runs the 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
• 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 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
• Vehicle Speed Sensor
• Brake switch
• Inlet 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