Engine Controls - Self-Diagnostics - 2.4L (K24DE) & 3.3L (VG33E): Overview Nissan Frontier D22

Description

As part of an enhanced emissions test for inspection and maintenance, certain states require the status of System Readiness Test (SRT) be used to indicate whether the ECM has completed the self-diagnosis of major emission systems and components. Completion must be verified in order for the emissions inspection to proceed.

If a vehicle is rejected for a state emissions inspection due to one or more SRT items indicating INCMP, use this information to set the SRT to CMPLT.

In most cases, the ECM will automatically complete its self-diagnosis cycle during normal usage, and SRT status will indicate CMPLT for each application system. Once set as CMPLT, the SRT status remains CMPLT until the self-diagnosis memory is erased.

Occasionally, certain potions of the self-diagnostic test may not be completed as a result of the customer's normal driving pattern. The SRT will indicate INCMP for these items.

The ECM performs on/off control of HO2S1 heaters based on engine speed. HO2S1 heaters are turned off above 3000 RPM (on 2.4L models) or 3200 RPM (on 3.3L models).

The ECM controls on/off operation of HO2S2 heaters based on engine speed. On 3.3L models, when engine speed is more than 3200 RPM, HO2S2 heaters are turned off. On 2.4L models, HO2S2 heaters are on when engine is running.

The Mass Airflow (MAF) sensor is placed in the stream of intake air. It measures the intake flow rate by measuring a part of the entire intake flow. It consists of a hot film that is supplied with electric current from the ECM. The temperature of the hot film is controlled by the ECM a certain amount. The heat generated by the hot film is reduced as the intake air flows around it. The more air, the greater the heat loss. Therefore, the ECM must supply more electric current to maintain the temperature of the hot film as air flow increases. The ECM detects the air flow by means of this current change.

The Absolute Pressure (AP) sensor is built into ECM. The sensor detects ambient barometric pressure and sends a voltage signal to the microcomputer.

The Intake Air Temperature (IAT) sensor is mounted to the air duct housing. It detects intake air temperature and transmits a signal to the ECM. The temperature sensing unit uses a thermistor which is sensitive to the change in temperature. Electrical resistance of the thermistor decreases in response to temperature increase.

The Engine Coolant Temperature (ECT) sensor is used to detect the engine coolant temperature. The sensor modifies a voltage signal from the ECM. The modified signal returns to the ECM as the engine coolant temperature input. The sensor uses a thermistor which is sensitive to the change in temperature. The electrical resistance of the thermistor decreases as temperature increases.

The TP sensor responds to the accelerator pedal movement. This sensor is a type of potentiometer which transforms the throttle position into output voltage, and emits the voltage signal to the ECM. In addition, the sensor detects the opening and closing speed of the throttle valve and feeds the voltage signal to the ECM.

Idle position of the throttle valve is determined by the ECM receiving the signal from the TP sensor. This sensor controls engine operation such as fuel cut. The wide open and closed throttle position switch, which is built into the TP sensor unit, is not used for engine control.

The Engine Coolant Temperature (ECT) sensor is used to detect the engine coolant temperature. The sensor modifies a voltage signal from the ECM. The modified signal returns to the ECM as the engine coolant temperature input. The sensor uses a thermistor which is sensitive to the change in temperature. The electrical resistance of the thermistor decreases as temperature increases.

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in the exhaust gas compared to the outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in the exhaust gas compared to the outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

To judge the malfunction of HO2S1, this diagnosis measures response time of HO2S1 signal. The time is compensated by engine operating (speed and load), fuel feedback control constant, and HO2S1 temperature index. Judgment is based on whether the compensated time (HO2S1 cycling time index) is inordinately long or not.

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in the exhaust gas compared to the outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

Under the condition in which the front HO2S signal is not input, the ECM circuits will read about .3 volt continuously. Therefore, for this diagnosis, the time that output voltage is 200-400 millivolts is monitored, and the diagnosis checks that this time is not inordinately long.

HO2S2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from HO2S2. This sensor is made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. Under normal conditions, HO2S2 is not used for engine control. HO2S2 has a much longer switching time between rich and lean than HO2S1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time.

To judge the malfunctions of HO2S2, ECM monitors whether the voltage of sensor is unusually high during the various driving condition such as fuel-cut.

HO2S2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from HO2S2. This sensor is made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. Under normal conditions, HO2S2 is not used for engine control. HO2S2 has a much longer switching time between rich and lean than HO2S1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time.

To judge the malfunctions of HO2S2, ECM monitors whether the switching response of sensor's voltage is faster than specified during the various driving condition such as fuel-cut.

With the air/fuel mixture ratio self-learning control, the actual mixture can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the front Heated Oxygen Sensor (HO2S). The ECM calculates the necessary compensation to correct the offset between the actual and theoretical ratios.

When the amount of compensation is extremely large (the actual mixture is too lean or too rich), the ECM determines that the fuel injection system is malfunctioning and turns on the MIL (2-trip detection logic).

The Fuel Tank Temperature (FTT) sensor is used to detect the fuel temperature inside the fuel tank. The sensor modifies a voltage signal from the ECM. The modified signal returns to the ECM as the fuel temperature input. The sensor uses a thermistor which is sensitive to the change in temperature. The electrical resistance of the thermistor decreases as temperature increases.

This diagnosis checks whether the engine coolant temperature is too high, even when engine load is not heavy. When malfunction is detected, MIL will light up on the 1st trip.

When a misfire occurs, engine speed will fluctuate. If the engine speed fluctuates enough to cause the Crankshaft Position (CKP) sensor signal to vary, ECM can detect a misfire. The misfire detection logic consists of the following two conditions.

1. One Trip Detection Logic (3-Way Catalyst Damage) On the first trip that a misfire condition occurs that can damage the 3-way catalyst due to overheating, the MIL will blink. When a misfire condition occurs, the ECM monitors the CKP sensor signal every 200 engine revolutions for a change. When the misfire condition decreases to a level that will not damage the catalyst, the MIL will turn off. If another misfire condition occurs that can damage the catalyst on a second trip, the MIL will blink. When the misfire condition decreases to a level that will not damage the catalyst, the MIL will remain on. If another misfire condition occurs that can damage the catalyst, the MIL will begin to blink again.
2. Two Trip Detection Logic (Exhaust Quality Deterioration) For misfire conditions that will not cause damage to the catalyst (but will affect vehicle emissions), the MIL will only light when the misfire is detected on a second trip. During this condition, the ECM monitors the CKP sensor signal every 1000 engine revolutions. A misfire malfunction can be detected on any one cylinder or on multiple cylinders.

The knock sensor is attached to the cylinder block. It senses engine knocking using a piezoelectric element. A knocking vibration from the cylinder block is sensed as vibrational pressure. This pressure is converted into a voltage signal and sent to the ECM.

Note. Freeze frame data will not be stored in ECM and MIL will not illuminate for the knock sensor.

The Crankshaft Position (CKP) Sensor (OBD) is located on the transaxle housing facing the gear teeth (cogs) of the flywheel or drive plate. It detects the fluctuation of the engine revolution.

The sensor consists of a permanent magnet and hall integrated circuit. When the engine is running, the high and low parts of the teeth cause the gap with the sensor to change. The changing gap causes the magnetic field near the sensor to change. Due to the changing magnetic field, the voltage from the sensor changes.

The ECM receives the voltage signal and detects the fluctuation of the engine revolution. This sensor is not directly used to control the engine system. It is used only for on-board diagnosis.

The Camshaft Position (CMP) sensor is a basic component of the engine control system. It monitors engine speed and piston position. It monitors engine speed and piston position. These input signals to the engine control system are used to control fuel injection, ignition timing and other functions.

The CMP sensor has a rotor plate and a wave-forming circuit. The rotor plate has 360 slits for a one-degree signal and 4 slits for a 180-degree signal (on 2.4L models) or 6 slits for a 120-degree signal (on 3.3L models). The wave-forming circuit consists of LEDs and photo diodes.

The rotor plate is positioned between the LED and the photo diode. The LED transmits light to the photo diode. As the rotor plate turns, the slits cut the light to generate rough-shaped pulses. These pulses are converted to on-off signals by the wave-forming circuit and sent to the ECM.

The distributor is not repairable and must be replaced as an assembly except for distributor cap and rotor.

This system cuts and controls vacuum applied to the EGR valve to suit engine operating conditions. This cut-and-control operation is accomplished through the ECM and the EGR Control (EGRC) solenoid valve. When the ECM detects any of the following conditions, current flowing through the solenoid valve is cut. This causes the vacuum to be discharged into the atmosphere. The EGR valve remains closed.

1. Low engine coolant temperature.
2. Engine starting.
3. High-speed engine operation.
4. Engine idling.
5. Excessively high engine coolant temperature.
6. Mass airflow sensor malfunction.
7. Low intake air temperature.

The EGR Control Backpressure Transducer (EGRC-BPT) valve monitors exhaust pressure to activate the diaphragm, controlling throttle body vacuum applied to the EGR valve. In other words, recirculated exhaust gas is controlled in response to positioning of the EGR valve or to engine operation.

If too much EGR flow exists due to an EGRC-BPT valve malfunction, off idle engine roughness will increase. If the roughness is large, then the vacuum to the EGR valve is interrupted through the EGRC solenoid valve. If the engine roughness is reduced at that time, the EGRC-BPT valve malfunction is indicated.

The EGR temperature sensor detects temperature changes in the EGR passageway. When the EGR valve opens, hot exhaust gases flow, and the temperature in the passageway changes. The EGR temperature sensor is a thermistor that modifies a voltage signal sent from the ECM. This modified signal then returns to the ECM as an input signal. As the temperature increases, EGR temperature sensor resistance decreases. This sensor is not used to control the engine system. It is only used for on-board diagnosis.

The ECM monitors the switching frequency ratio of HO2S1 and HO2S2. A 3-way catalyst with high oxygen storage capacity will indicate a low switching frequency of HO2S2. As oxygen storage capacity decreases, HO2S2 switching frequency will increase. When the frequency ratio of HO2S1 and HO2S2 approaches a specified limit value, the 3-way catalyst malfunction is diagnosed.

In this EVAP control system, purge flow occurs during non-closed throttle conditions. Purge volume is related to air intake volume. Under normal purge conditions (non-closed throttle), the EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV) is open to admit purge flow. Purge flow exposes the EVAP Control System Pressure Sensor (EVAP-CSPS) to intake manifold vacuum.

Under normal purge conditions (non-closed throttle), sensor output voltage indicates if pressure drop and purge flow are adequate. If not, a fault is determined.

This diagnosis detects leaks in the EVAP purge line using engine intake manifold vacuum. If pressure does not increase, the ECM will check for leaks in the line between the fuel tank and EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV), under the following "vacuum test" conditions.

The vacuum cut valve bypass valve is opened to clear the line between the fuel tank and the EVAP-CPVCSV. The EVAP Canister Vent Control Valve (EVAP-CVCV) will then be closed to shut the EVAP purge line off. The EVAP-CPVCSV is opened to depressurize the EVAP purge line using intake manifold vacuum. After this occurs, the EVAP-CPVCSV will be closed.

This system controls flow rate of fuel vapor from the EVAP canister. The opening of the vapor by-pass passage in the EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV) changes to control the flow rate. The EVAP-CPVCSV repeats ON/OFF operation according to the signal sent from the ECM. The opening of the valve varies for optimum engine control. The optimum value stored in the ECM is determined by considering various engine conditions. When the engine is operating, the flow rate of fuel vapor from the EVAP canister is regulated as the air flow changes.

The EVAP-CPVCSV uses an ON/OFF duty to control the flow rate of fuel vapor from the EVAP canister. The EVAP-CPVCSV is moved by ON/OFF pulses from the ECM. The longer the ON pulse, the greater the amount of fuel vapor that will flow through the valve.

The EVAP Canister Vent Control Valve (EVAP-CVCV) is located on the EVAP canister and is used to seal the canister vent. This solenoid valve responds to signals from the ECM. When the ECM sends an ON signal, the coil in the solenoid valve is energized. A plunger will then move to seal the canister vent. The ability to seal the vent is necessary for the on board diagnosis of other evaporative emission control system components. This solenoid valve is used only for diagnosis, and usually remains opened. When the vent is closed, under normal purge conditions, the evaporative emission control system is depressurized and allows "EVAP Control System (Small Leak)" diagnosis.

The EVAP Control System Pressure Sensor (EVAP-CSPS) detects pressure in the purge line. The sensor output voltage to ECM increases as pressure increases. The EVAP-CSPS is not used for engine control. It is only used for on-board diagnosis.

This diagnosis detects a very large leak (fuel filler cap fell off, etc.) in EVAP system between the fuel tank and EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV).

This diagnosis detects very small leaks in the EVAP purge line between the fuel tank and EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV) using engine intake manifold vacuum. If ECM detects a very small leak, DTC P0456 will set. If ECM detects a small leak, DTC P0442 will set.

The fuel level sensor is mounted in the fuel level sensor unit. The sensor detects a fuel level in the fuel tank and transmits a signal to the ECM. It consists of 2 parts, one is mechanical float and the other side is variable resistor. Fuel level sensor output voltage changes depending on the movement of the fuel mechanical float.

When the vehicle is parked, naturally the fuel level in the fuel tank is stable. It means that output signal of the fuel level sensor does not change. If ECM senses sloshing signal from the sensor, fuel level sensor malfunction is detected. Malfunction is detected when even though the vehicle is parked, a signal being varied is sent from the fuel level sensor to ECM.

The fuel level sensor is mounted in the fuel level sensor unit. The sensor detects a fuel level in the fuel tank and transmits a signal to the ECM. It consists of 2 parts, one is mechanical float and the other side is variable resistor. Fuel level sensor output voltage changes depending on the movement of the fuel mechanical float.

Driving long distances naturally affects fuel gauge level. This diagnosis detects the fuel gauge malfunction of the gauge not moving even after a long distance has been driven.

The fuel level sensor is mounted in the fuel level sensor unit. The sensor detects a fuel level in the fuel tank and transmits a signal to the ECM. It consists of 2 parts, one is mechanical float and the other side is variable resistor. Fuel level sensor output voltage changes depending on the movement of the fuel mechanical float.

ECM receives 2 signals from the fuel level sensor circuit. One is fuel level sensor power supply circuit, and the other is fuel level sensor ground circuit. This diagnosis indicates the former, to detect open or short circuit malfunction.

The Vehicle Speed Sensor (VSS) is installed in the transmission. It contains a pulse generator which provides a vehicle speed signal to the instrument cluster. The instrument cluster then sends a signal to the ECM.

This system automatically controls engine idle speed to a specified level. Idle speed is controlled through fine adjustment of the amount of air which bypasses the throttle valve via Idle Air Control Valve-Auxiliary Air Control (IACV-AAC) valve. The IACV-AAC valve repeats on-off operation according to the signal sent from the ECM. The Camshaft Position (CMP) sensor detects the actual engine speed and sends a signal to the ECM. The ECM then controls the on-off time of the IACV-AAC valve so that engine speed coincides with the target value memorized in ECM. The target engine speed is the lowest speed at which the engine can operate steadily. The optimum value stored in the ECM is determined by taking into consideration various engine conditions, such as during warm-up, deceleration, and engine load (A/C, power steering and cooling fan operation.

The IACV-AAC is moved by on-off pulses from the ECM. The longer the on pulse, the greater the amount of air that will flow through the valve. The more air that flows through the valve, the higher the idle speed.

A closed Throttle Position (TP) switch and wide open TP switch are built into the TP sensor unit. The wide open TP switch is used only for A/T control.

When the throttle valve is in the closed position, the closed TP switch sends a voltage signal to the ECM. The ECM only uses this signal to open or close the EVAP canister purge volume control solenoid valve when the TP sensor is malfunctioning.

These circuits are used to control the smooth shifting up and down during hard acceleration or deceleration. Voltage signals are exchanged between ECM and Transmission Control Module (TCM).

The ECM consists of a microcomputer and connector for signal input and output and for power supply.

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in the exhaust gas compared to the outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

To judge the malfunction, the output from HO2S1 is monitored to determine whether the "rich" output is sufficiently high and whether the "lean" output is sufficiently low. When both the outputs are shifting to the lean side, the malfunction will be detected.

The front Heated Oxygen Sensors (HO2S1) are located in each front exhaust pipe, upstream of catalyst. They detect the amount of oxygen in the exhaust gas compared to the outside air. The front HO2S has a closed-end tube made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. The front HO2S signal is sent to the ECM. The ECM adjusts the injection pulse duration to achieve the ideal air-fuel ratio. The ideal air-fuel ratio occurs near the radical change from one volt to zero volts.

To judge the malfunction, the output from HO2S1 is monitored to determine whether the "rich" output is sufficiently high and whether the "lean" output is sufficiently low. When both the outputs are shifting to the rich side, the malfunction will be detected.

HO2S2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from HO2S2. This sensor is made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. Under normal conditions, HO2S2 is not used for engine control.

HO2S2 has a much longer switching time between rich and lean than HO2S1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time. To judge the malfunctions of HO2S2, ECM monitors whether the minimum voltage of sensor is sufficiently low during the various driving condition such as fuel-cut.

HO2S2, located after 3-way catalyst, monitors the oxygen level in the exhaust gas on each bank. Even if switching characteristics of HO2S1 are shifted, the air fuel ratio is controlled to stoichiometric, by the signal from HO2S2. This sensor is made of ceramic zirconia. The zirconia generates voltage from about one volt in richer conditions to zero volts in leaner conditions. Under normal conditions, HO2S2 is not used for engine control.

HO2S2 has a much longer switching time between rich and lean than HO2S1. The oxygen storage capacity before the 3-way catalyst causes the longer switching time. To judge the malfunctions of HO2S2, ECM monitors whether the maximum voltage of sensor is sufficiently high during the various driving condition such as fuel-cut.

If the cooling fan or another component in the cooling system malfunctions, engine coolant temperature will rise. When the engine coolant temperature reaches an abnormally high temperature condition, a malfunction is indicated.

The Crankshaft Position (CKP) Sensor (OBD) is located on the transaxle housing facing the gear teeth (cogs) of the flywheel or drive plate. It detects the fluctuation of the engine revolution.

The sensor consists of a permanent magnet and hall integrated circuit. When the engine is running, the high and low parts of the teeth cause the gap with the sensor to change. The changing gap causes the magnetic field near the sensor to change. Due to the changing magnetic field, the voltage from the sensor changes.

The ECM receives the voltage signal and detects the fluctuation of the engine revolution. This sensor is not directly used to control the engine system. It is used only for on-board diagnosis.

The EGR Control (EGRC) solenoid valve responds to signals from the ECM. When the ECM sends an OFF signal, the vacuum signal passes through the solenoid valve. The signal then reaches the EGR valve. When the ECM sends an ON signal, a plunger will then move to cut the vacuum signal from the throttle body to the EGR valve.

This system cuts and controls vacuum applied to the EGR valve to suit engine operating conditions. This cut-and-control operation is accomplished through the ECM and the EGR Control (EGRC) solenoid valve. When the ECM detects any of the following conditions, current flowing through the solenoid valve is cut. This causes the vacuum to be discharged into the atmosphere. The EGR valve remains closed.

1. Low engine coolant temperature.
2. Engine starting.
3. High-speed engine operation.
4. Engine idling.
5. Excessively high engine coolant temperature.
6. Mass airflow sensor malfunction.
7. Low intake air temperature.

This diagnosis detects leaks in the EVAP purge line using vapor pressure in the fuel tank. The EVAP Canister Vent Control Valve (EVAP-CVCV) is closed to shut the EVAP purge line. The vacuum cut valve bypass valve will then be opened to clear the line between the fuel tank and the EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV). The EVAP Control System Pressure Sensor (EVAP-CSPS) can now monitor the pressure inside the fuel tank. If pressure increases, the ECM will check for leaks in the line between the vacuum cut valve and EVAP-CPVCSV.

This system controls flow rate of fuel vapor from the EVAP canister. The opening of the vapor by-pass passage in the EVAP Canister Purge Volume Control Solenoid Valve (EVAP-CPVCSV) changes to control the flow rate. The EVAP-CPVCSV repeats ON/OFF operation according to the signal sent from the ECM. The opening of the valve varies for optimum engine control. The optimum value stored in the ECM is determined by considering various engine conditions. When the engine is operating, the flow rate of fuel vapor from the EVAP canister is regulated as the air flow changes.

The EVAP-CPVCSV uses an ON/OFF duty to control the flow rate of fuel vapor from the EVAP canister. The EVAP-CPVCSV is moved by ON/OFF pulses from the ECM. The longer the ON pulse, the greater the amount of fuel vapor that will flow through the valve.

The EVAP Canister Vent Control Valve (EVAP-CVCV) is located on the EVAP canister and is used to seal the canister vent. This solenoid valve responds to signals from the ECM. When the ECM sends an ON signal, the coil in the solenoid valve is energized. A plunger will then move to seal the canister vent. The ability to seal the vent is necessary for the on board diagnosis of other evaporative emission control system components. This solenoid valve is used only for diagnosis, and usually remains opened. When the vent is closed, under normal purge conditions, the evaporative emission control system is depressurized and allows "EVAP Control System (Small Leak)" diagnosis.

The EVAP Canister Vent Control Valve (EVAP-CVCV) is located on the EVAP canister and is used to seal the canister vent. This solenoid valve responds to signals from the ECM. When the ECM sends an ON signal, the coil in the solenoid valve is energized. A plunger will then move to seal the canister vent. The ability to seal the vent is necessary for the on board diagnosis of other evaporative emission control system components. This solenoid valve is used only for diagnosis, and usually remains opened. When the vent is closed, under normal purge conditions, the evaporative emission control system is depressurized and allows "EVAP Control System (Small Leak)" diagnosis.

The fuel level sensor is mounted in the fuel level sensor unit. The sensor detects a fuel level in the fuel tank and transmits a signal to the ECM. It consists of 2 parts, one is mechanical float and the other side is variable resistor. Fuel level sensor output voltage changes depending on the movement of the fuel mechanical float.

ECM receives 2 signals from the fuel level sensor. One is fuel level sensor power supply circuit, and the other is fuel level sensor ground circuit. This diagnosis indicates the latter to detect open circuit malfunction.

The vacuum cut valve and vacuum cut valve by-pass valve are installed in parallel on the EVAP purge line between the fuel tank and the EVAP canister. The vacuum cut valve prevents the intake manifold vacuum from being applied to the fuel tank. The vacuum cut valve by-pass valve is a solenoid type valve and generally remains closed. It opens only for on board diagnosis. The vacuum cut valve by-pass valve responds to signals from the ECM. When the ECM sends an ON (ground) signal, the valve is opened. The vacuum cut valve is then by-passed to apply intake manifold vacuum to the fuel tank.

The vacuum cut valve and vacuum cut valve by-pass valve are installed in parallel on the EVAP purge line between the fuel tank and the EVAP canister. The vacuum cut valve prevents the intake manifold vacuum from being applied to the fuel tank. The vacuum cut valve by-pass valve is a solenoid type valve and generally remains closed. It opens only for on board diagnosis. The vacuum cut valve by-pass valve responds to signals from the ECM. When the ECM sends an ON (ground) signal, the valve is opened. The vacuum cut valve is then by-passed to apply intake manifold vacuum to the fuel tank.

The malfunction information related to A/T is transferred through the circuit from Transmission Control Module (TCM) to ECM.

When the gear position is "P" (A/T models only) or "N", Park/Neutral Position (PNP) switch is on. ECM detects the position because the continuity of the line (the "on" signal) exists.