THEORY OF OPERATION
The Secondary Air Injection System reduces hydrocarbon emissions by decreasing the catalyst warm up time. During a cold engine start, after a short diagnostic time out, typically two to three seconds, the Powertrain Control Module (PCM) commands the Secondary Air Pump on for approximately 20 seconds. The Secondary Air Pump supplies air to the exhaust manifold through the Secondary Air Pump Valve, a one-way check valve. This additional air introduced to the exhaust flow aids in the warm up of the catalyst.
The catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst. As a catalyst ages, it loses some of its oxygen storing capacity. As a result, part of the untreated exhaust gases can break through the catalyst and causes the downstream O2 sensor to deviate from its neutral (Stoichiometric) position. By observing the downstream O2 signal, the degradation level of catalyst can be detected. In general, the higher the downstream O2 sensor state of change value, the more exhaust gas breakthrough and the lower the oxygen storage capacity of the catalytic converter.
The Powertrain Control Monitor (PCM) detected that the EVAP system was unable to achieve or maintain vacuum during the test period or the system was not able to close the ESIM switch when vacuum was present.
The Evaporative Purge Monitor tests the integrity of the hoses/tube between the throttle body/intake and the fuel tank. The monitor will be enabled under either a Small Leak test or it will run during a Large Leak test (This is when Small Leak test does not pass). During a Small Leak test, the monitor will first evaluate the delta pressure on the Fuel Tank Pressure (FTP) sensor while normal purge control is active. If the monitor does not pass within a calibrated amount of time, then an intrusive monitor will be enabled. This intrusive monitor will ramp in the purge flow to a target amount while evaluating the delta pressure in the entire system. If the delta pressure between purge off and purge on exceeds a calibrated amount, then the monitor will ramp out the purge flow and evaluate the delta pressure between the high flow and the new low flow target. If the delta pressure is less than a calibrated threshold then the monitor will pass.
The Evaporative Purge Monitor tests the integrity of the hoses/tube between the throttle body/intake and the fuel tank. The monitor is a two stage test and runs only after the Evaporative system passes the small leak test. Stage one is non-intrusive. The Powertrain Control Module (PCM) monitors the purge vapor ratio and the Evaporative System Integrity Monitor (ESIM) switch closed ratio. If the purge vapor ratio is above a calculated value, the monitor passes. If the ESIM switch closed ratio is greater than calculated value when purge flow is greater than a minimum value, the monitor passes. Stage two is an intrusive test and runs only if stage one does not pass. The PCM commands the purge solenoid to flow at a specified rate to force the purge vapor ratio to update. The ratio is compared to a calibrated specification. If it is less than specified, a one trip failure is recorded. This test can detect if the purge hose is off, obstructed or the purge valve is not operational.
This diagnostic tests the evaporative emission (EVAP) system for a small leak when the ignition is turned OFF and the correct conditions are met. Heat is transferred from operating the vehicle and ambient conditions into a vehicle fuel tank during normal operation. When the ignition is turned OFF and the EVAP system is sealed, a change in the fuel tank vapor temperature occurs, which results in corresponding pressure changes in the fuel tank vapor space. This change is monitored by the Powertrain Control Module (PCM) using the fuel tank pressure sensor input. The PCM then makes a judgment on the integrity of the system. With a 0.51 mm (0.02 in) leak in the system, the amount of pressure change observed is significantly less than that of a sealed system.
Fuel level is recorded when the ignition key is turned off and is compared to the fuel level when the ignition key is turned back on. The Powertrain Control Module (PCM) recognizes an increase in fuel level and will fail the Large leak test because the fuel cap is broken or not installed properly. GAS CAP will be displayed to inform the owner that the cap is off of loose.
For 20 seconds following a vehicle cold start, the air injection system delivers a controlled amount of outside air into the exhaust system, upstream of the catalytic converters, in order to help reduce hydrocarbon and carbon monoxide gas emissions.
The vehicle speed sensor rationality is a continuous test that monitors the vehicle speed sensor for lack of activity. The rationality will not run if a limp-in exists for MAP, Throttle Position, and Engine Coolant Temperature. If vehicle speed sensor is below a minimum threshold for a period of time after the vehicle is operated at a sufficient load, a failure will be indicated.
The vehicle speed sensor rationality is a continuous test that monitors the vehicle speed sensor for lack of activity. The rationality will not run if a limp-in exists for MAP, Throttle Position, and Engine Coolant Temperature. If vehicle speed sensor is below a minimum threshold for a period of time after the vehicle is operated at a sufficient load, a failure will be indicated.
The objective of the Idle Speed Rationality monitor is to evaluate the ability of the idle speed control system to achieve and maintain a steady idle, by monitoring engine RPM during idle. If engine RPM is not within a calibrated deadband of target idle speed, a timer is started. If the timer reaches a maximum threshold without any sign of the RPM trending towards control, a fault is generated.
The objective of the Idle Speed Rationality monitor is to evaluate the ability of the idle speed control system to achieve and maintain a steady idle, by monitoring engine RPM during idle. If engine RPM is not within a calibrated deadband of target idle speed, a timer is started. If the timer reaches a maximum threshold without any sign of the RPM trending towards control, a fault is generated.
Spark adjustment during a cold start is intended to provide quick response to idle speed variations. The Powertrain Control Module (PCM) monitors spark advance on a cold start over a period of time, then compares the average spark advance to a threshold.
The A/C pressure transducer monitors the pressures in the high side of the A/C refrigerant system through its connection to a fitting on the A/C discharge line and its internal resistance changes in response to the pressures it monitors. The Totally Integrated Power Module (TIPM) provides a five Volt reference signal and a sensor ground to the A/C pressure transducer, then monitors the output voltage of the transducer on a sensor return circuit to determine refrigerant pressure the TIPM busses the reads to PCM. The PCM is programmed to respond to this and other sensor inputs by controlling the operation of the A/C clutch and the radiator cooling fan to help optimize A/C system performance and to protect the A/C system components from damage. The PCM will request disengagement of the A/C clutch when high side pressure rises above 3219 kPa (476 psi) and re-engage the clutch when high side pressure drops below 2937 kPa (426 psi). The PCM will also disengage the A/C clutch if the high side pressure drops below 110 kPa (16 psi) and will re-engage the clutch when the high side pressure rises above 220 kPa (32 psi). When the refrigerant pressure rises above 1655 kPa (240 psi), the PCM will actuate the cooling fan.
The A/C pressure transducer monitors the pressures in the high side of the A/C refrigerant system through its connection to a fitting on the A/C discharge line and its internal resistance changes in response to the pressures it monitors. The Totally Integrated Power Module (TIPM) provides a five Volt reference signal and a sensor ground to the A/C pressure transducer, then monitors the output voltage of the transducer on a sensor return circuit to determine refrigerant pressure the TIPM busses the reads to PCM. The PCM is programmed to respond to this and other sensor inputs by controlling the operation of the A/C clutch and the radiator cooling fan to help optimize A/C system performance and to protect the A/C system components from damage. The PCM will request disengagement of the A/C clutch when high side pressure rises above 3219 kPa (476 psi) and re-engage the clutch when high side pressure drops below 2937 kPa (426 psi). The PCM will also disengage the A/C clutch if the high side pressure drops below 110 kPa (16 psi) and will re-engage the clutch when the high side pressure rises above 220 kPa (32 psi). When the refrigerant pressure rises above 1655 kPa (240 psi), the PCM will actuate the cooling fan.