Monitored Systems for Emission Controls
Monitored Systems for Emission Controls
Monitored System
There are new electronic circuit monitors that check fuel, emissions, engine and ignition performance. These monitors use information from various sensor circuits to indicate the overall operation of the fuel, engine, ignition and emission systems and thus the emissions performance of the vehicle.
The fuel, engine, ignition and emission system monitors do not indicate a specific component problem. They do indicate that there is an implied problem within one of the systems and that a specific problem must be diagnosed.
If any of these monitors detect a problem affecting vehicle emissions, the malfunction indicator lamp will be illuminated. These monitors generate diagnostic trouble codes that can be displayed with a scan tool.
The following is a list of the system monitors:
• Exhaust gas recirculation monitor, if equipped.
• Misfire monitor.
• Fuel system monitor.
• Oxygen sensor monitor.
• Oxygen sensor heater monitor.
• Catalyst monitor.
• Evaporative system leak detection monitor, if equipped.
The following is a description of each system monitor, and its DTC.
Oxygen Sensor Monitor
Effective control of the exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2 sensor. The O2 sensor is located in the exhaust path. Once it reaches operating temperatures of 300° to 350 °C (572° to 662 °F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. The powertrain control module is programmed to maintain the optimum air/fuel ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons, carbon monoxide and oxides of nitrogen from the exhaust.
The O2 sensor is also the main sensing element for the EGR (if equipped), catalyst and fuel monitors.
The O2 sensor may fail in any or all of the following manners:
• Slow response rate.
• Reduced output voltage.
• Dynamic shift.
• Shorted or open circuits.
The response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum air/fuel mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas.
The output voltage of the O2 sensor ranges from 0 to 1 volt (voltages are offset by 2.5 volts on NGC vehicles). 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.
Oxygen Sensor Heater Monitor
If there is an O2 sensor DTC as well as a O2 sensor heater DTC, the O2 sensor heater fault must be repaired first. After the O2 sensor fault is repaired, verify that the heater circuit is operating correctly.
Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2 sensor. The O2 sensor is located in the exhaust path. Once it reaches operating temperatures of 300° to 350 °C (572 ° to 662 °F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 air fuel ratio. At this mixture ratio, the catalyst works best to remove the HC, CO and NOX from the exhaust.
The voltage readings taken from the O2 sensor are very temperature sensitive. The readings are not accurate below 300 °C (572 °F). Heating of the O2 sensor is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2 sensor must be tested to ensure that it is heating the sensor properly.
The O2 sensor circuit is monitored for a drop in voltage. The sensor output is used to test the heater by isolating the effect of the heater element on the O2 sensor output voltage from the other effects.
EGR Monitor (If Equipped)
The PCM performs an on-board diagnostic check of the EGR system.
The EGR monitor is used to test whether the EGR system is operating within specifications. The diagnostic check activates only during selected engine/driving conditions. When the conditions are met, the EGR is turned off (solenoid energized) and the O2 sensor compensation control is monitored. Turning off the EGR shifts the air fuel ratio in the lean direction. The O2 sensor data should indicate an increase in the O2 concentration in the combustion chamber when the exhaust gases are no longer recirculated. While this test does not directly measure the operation of the EGR system, it can be inferred from the shift in the O2 sensor data whether the EGR system is operating correctly. Because the O2 sensor is being used, the O2 sensor test must pass its test before the EGR test. Also looks at EGR linear potentiometer for feedback.
Misfire Monitor
Excessive engine misfire results in increased catalyst temperatures and causes an increase in HC emissions. Severe misfires could cause catalyst damage. To prevent catalytic converter damage, the PCM monitors engine misfire.
The PCM monitors for misfire during most engine operating conditions (positive torque) by looking at changes in the crankshaft speed. If a misfire occurs the speed of the crankshaft will vary more than normal.
Fuel System Monitor
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emission of HC, CO and NOX. The catalyst works best when the air fuel ratio is at or near the optimum of 14.7 to 1.
The PCM is programmed to maintain the optimum air/fuel ratio. This is done by making short term corrections in the fuel injector pulse width based on the O2 sensor output. The programmed memory acts as a self calibration tool that the engine controller uses to compensate for variations in engine specifications, sensor tolerances and engine fatigue over the life span of the engine. By monitoring the actual air fuel ratio with the O2 sensor (short term) and multiplying that with the program long term (adaptive) memory and comparing that to the limit, it can be determined whether it will pass an emissions test. If a malfunction occurs such that the PCM cannot maintain the optimum air/fuel ratio, then the MIL will be illuminated.
Catalyst Monitor
To comply with clean air regulations, vehicles are equipped with catalytic converters. These converters reduce the emission of HC, CO and NOX.
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 O2 sensors to monitor the efficiency of the converter. The dual O2 sensors 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 O2 sensor is used to detect the amount of oxygen in the exhaust gas before the gas enters the catalytic converter. The PCM calculates the air/fuel mixture from the output of the O2 sensor. A low voltage indicates high oxygen content (lean mixture). A high voltage indicates a low content of oxygen (rich mixture).
When the upstream O2 sensor detects a lean 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 O2 sensor 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 O2 sensor. 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 sensor copies the voltage of the upstream sensor. 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 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 will be illuminated.