Monitored Components
DESCRIPTION - MONITORED COMPONENTS
There are several components that will affect exhaust emissions if they malfunction. If the PCM (1) detects that one of these components has malfunctioned, the Malfunction Indicator Lamp (MIL) will illuminate. While the engine is running, the PCM (1) Monitors the following components for normal operation:
- Air Injection Motor (2)
- Air Switchover Valve (3)
- Left Air Shutoff Valve (4)
- Accelerator Pedal Position Sensor (5)
- Fuel Injectors (6)
- Right Air Shutoff Valve (7)
- Manifold Absolute Pressure (MAP) Sensor (8)
- Mass Airflow (MAF) Sensor (10)
- Throttle Valve Actuator/Throttle Position Sensor (TPS) (11)
- Ignition Coils (12)
- Crankshaft Position Sensor (13)
- Left Catalytic Converter (14)
- Left Upstream Oxygen (02) Sensor (15)
- Left Downstream 02 Sensor (16)
- Right Upstream 02 Sensor (17)
- Right Catalytic Converter (18)
- Right Downstream 02 Sensor (19)
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). The PCM monitors components to ensure they are working within specification. This is done by watching for a Electronic Throttle Control/TPS indication of a greater or lesser throttle opening than Manifold Absolute Pressure (MAP) and engine rpm indicate.
The following is a list of the monitored components:
- Catalyst Monitor
- Comprehensive Components
- Air Injection System/Catalyst startup heating
- Fuel Control (rich/lean)
- Oxygen Sensor Monitor
- Oxygen Sensor Heater Monitor
- Purge
- Misfire
COMPREHENSIVE COMPONENTS
Along with the major monitors, OBD II requires that the vehicle's on-board diagnostic system monitor any component that could affect emissions levels.
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 Component monitoring include:
- Circuit Continuity
- Open
- Shorted to Voltage
- Shorted to Ground
- Rationality and Functionality
- Outputs Tested for functionality
NOTE: Comprehensive component monitors are continuous. Therefore, enabling conditions do not apply. All will set a DTC and illuminate the MIL in 1 trip.
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 and CAN Bus 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
- Crankshaft Position (CKP) Sensor
- Mass Air Flow (MAF)/Intake Air Temperature (IAT) Sensor
- Accelerator Pedal Position Sensor (APPS)
- Electronic Throttle Control/Throttle Position Sensor (TPS)
- Knock Sensors
- Oxygen Sensor Heater
- PCM
- P/N Switch
- Transmission Control Module
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 Electronic Throttle Control (ETC) 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
- Air Pump Switchover Solenoid
- Manifold Flow Valve Solenoid
- Manifold Swirl Valve Solenoid
- Ignition Coils
- Throttle Body (Electronic Throttle Control/Throttle Position Sensor)
- Purge Solenoid
- Transmission Controls
OXYGEN SENSOR (O2S) MONITOR
DESCRIPTION - Effective control of exhaust emissions is achieved by an oxygen sensor feedback system. The most important element of the feedback system are the oxygen (O2) sensors. The O2 sensors are located in the exhaust path. Once they reach operating temperature 300° to 350° C (572° to 662° F), the sensors generate 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 sensors produce a low voltage, below 450mV. When the oxygen content is lower, caused by a rich condition, the sensors produce a higher voltage, above 450mV.
The exhaust oxygen levels detected by the sensors 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 is most effective at oxidizing (burning off) hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NOx) gasses from the exhaust.
The O2 sensors are also the main sensors that the PCM uses to monitor the EVAP Purge System, and Catalyst and Fuel Monitors.
The O2S may fail in any or all of the following manners:
- Slow response rate
- Reduced output voltage
- Heater Performance
- Shorted or open circuits
Slow Response Rate - 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 air/fuel mixture or vice versa. The PCM checks the oxygen sensor voltage in increments of a few milliseconds. 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.
Reduced Output Voltage - 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 air/fuel 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.
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 2 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.
OXYGEN SENSOR HEATER MONITOR
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.
Note: The O2S Heaters are kept off at coolant temperatures below 20° C (68° F) and at high engine rpm in order to avoid damaging the heaters.
The voltage reading taken from the O2S are very temperature sensitive. The readings taken from the O2S are not accurate below 300° C (572° F). 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. The heater resistance is checked by the PCM 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, shorted high or shorted low circuit.
OPERATION - The Oxygen Sensor Heater Monitor begins after the ignition has been turned OFF and the O2 sensors have cooled. As the sensor cools down, the resistance increases and the PCM reads the increase in voltage. Once voltage has increased to a predetermined amount, higher than when the test started, the oxygen sensor is cool enough to test heater operation.
When the oxygen sensor is cool enough, the PCM provides a ground path for the O2S heater circuit. Voltage to the O2 sensor begins to increase the temperature. As the sensor temperature increases, the internal resistance decreases.
The heater elements are tested each time the engine is turned OFF if all the enabling conditions are met. If the monitor fails, the PCM stores a maturing fault and a Freeze Frame is entered. If two consecutive tests fail, a DTC is stored. Because the ignition is OFF, the MIL is illuminated at the beginning of the next key cycle, after the 2nd failure.
CATALYST MONITOR
DESCRIPTION - 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 restriction of the exhaust. This can increase vehicle emissions and deteriorate engine performance, driveability and fuel economy.
The catalyst monitor uses four oxygen sensors (O2 sensors) to monitor the efficiency of the catalytic converters. The four O2 sensor strategy is based on the fact that as catalyst elements deteriorate, their oxygen storage capacity and their oxidizing (burning) efficiency are both reduced. By monitoring the oxygen storage capacity of a catalyst, its efficiency can be indirectly calculated. The upstream O2 sensors are used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converters. The PCM calculates the air/fuel mixture from the output of the O2 sensors. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2 sensors detect a high oxygen condition, there is an abundance of oxygen in the exhaust gas. An efficiently functioning catalytic converter would store this oxygen so it can be used for the oxidation of HC and CO gasses in the exhaust. As the converters absorb the oxygen, there will be a lack of oxygen downstream of the converters. The output of the downstream O2S will indicate limited activity in this condition.
As the converters lose the ability to store oxygen, and become less efficient, the condition can be detected from the behavior of the downstream O2 sensors. 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 O2 sensors will be virtually the same as the voltage of the upstream sensors. The only difference is a time lag (seen by the PCM) between the switching of the O2 sensors.
To monitor the system, the number of lean-to-rich switches of upstream and downstream O2 sensors are 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 is occurring in the catalytic converter.
The system must be monitored so that when catalyst efficiency deteriorates and exhaust emissions increase to over the legal limit, the MIL will be illuminated.
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 O2S 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 O2S switches is divided by the number of Front O2S switches to determine the switching ratio.
The test runs for 20 seconds. As catalyst efficiency deteriorates 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 record of the engine operating conditions is stored in the PCM's memory. 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 turned off after three consecutive good trips.