You can diagnose all of the sensors and most of the input circuits with a scan tool. Within this section is a short description of how to use a scan tool wherever possible to diagnose these circuits. You can also use the scan tool to compare the values for an engine that is running normally with the engine you are diagnosing.

The A/C request circuit signals the ECM when you select an A/C mode at the A/C control head. The ECM uses this information in order to enable the A/C compressor clutch and in order to adjust the idle speed before turning ON the A/C clutch. If this signal is not available to the ECM, the A/C compressor will be inoperative.

Refer to HVAC Compressor Clutch Circuit Diagnosis for A/C wiring diagrams and diagnosis of A/C electrical system.

The A/C Load circuit signals the ECM when the A/C compressor system is under an excessive load during, for example, any high ambient conditions. If the system pressure exceeds 1,200 kPa, the ECM will detect this condition and increase the idle speed by about 50 RPM in order to compensate for the excess load.

Refer to HVAC Compressor Clutch Circuit Diagnosis for A/C wiring diagrams and for diagnosis of the A/C electrical system.

The knock sensor (KS) system detects engine detonations. The ECM will retard the spark timing based on the signals from the KS module. The knock sensors produce an AC voltage that is sent to the KS module. The amount of the AC voltage produced is proportional to the amount of knock.

The control module reads the voltage of the sensor during the 45 degrees after cylinder 2,4, or 6 has fired and the voltage of the sensor during the 45 degrees after cylinder 1, 3, or 5 has fired.

If knock occurs in any of the cylinders, the ignition will be retarded by 3 degrees for that particular cylinder. If the knocking then stops, the ignition will be restored to what it was before in steps of 0.75 degrees.

Should knocking continue in the same cylinder in spite of the ignition being retarded, the control module will retard the ignition an additional step of 3 degrees, and so on, up to a maximum of 12.75 degrees. The ignition will also be retarded at high ambient temperatures in order to counteract knocking tendencies provoked by high intake air temperatures.

Should either the bank 1 or the bank 2 sensor fail to work, or should a break in the circuit occur (no continuity), the ignition timing will then use a default strategy that will retard the ignition much more than normal.

The Camshaft Position (CMP) sensor works in conjunction with a single tooth reluctor wheel on the Bank 2 Intake camshaft. The ECM pulls up the CMP sensor signal circuit to 12 volts and monitors this voltage. As the reluctor wheel tooth rotates past the sensor, the sensor's internal circuitry pulls the signal circuit to ground, creating a square wave signal used by the ECM. The reluctor wheel tooth covers 180 degrees of the camshaft circumference. This causes the CMP signal voltage to transition once per crankshaft revolution. This signal, when combined with the CKP sensor signal, enables the ECM to determine exactly which cylinder is on a firing stroke. The ECM can then properly synchronize the ignition system, the fuel injectors and the knock control. Note that as long as the CKP signal is available, the engine can start even if there is no CMP sensor signal. The ECM will default to non-sequential fuel injector operation.

The ECM also monitors the CKP sensor system for malfunctions. The DTC P0340 - CMP Sensor Circuit indicates that the ECM has detected a CMP system problem.

The Camshaft Position (CMP) sensor signal is used to determine which of the two cylinders is on a firing stroke. The ECM can then properly synchronize the ignition system, the fuel injectors, and the knock control. This sensor is also used to detect a misfire. Refer to DTC P0300 Engine Misfire Detected for information on misfire detection.

The ECM also monitors the CKP sensor system for malfunctions. The following DTC indicates that the ECM has detected a CKP system problem: DTC P0335 Crankshaft Position (CKP) Sensor Circuit

The Engine Coolant Temperature (ECT) sensor (1) contains a semiconductor device that changes resistance based on temperature (a thermistor). The ECT sensor is located in the coolant crossover pipe at the center rear of the engine. The ECT sensor has a signal circuit and a ground circuit. The ECM applies a voltage to the sensor (about 5.0 volts) on the signal circuit. The ECM monitors changes in this voltage caused by changes in the resistance of the sensor in order to determine the engine coolant temperature.

When the engine coolant is cold, the sensor (thermistor) resistance is high, and the ECM's signal voltage is only pulled down a small amount through the sensor to ground. When this occurs, the ECM senses a high signal voltage (low temperature). When the engine coolant is warm, the sensor's resistance is low, and the signal voltage is pulled down a greater amount. This causes the ECM to sense a low signal voltage (high temperature).

This voltage is used to determine:

The scan tool displays engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. When the ECM detects a malfunction in the ECT sensor or circuit, the following DTC(s) will set:

The fuel tank pressure sensor mounts to the sending unit at the top of the fuel tank. The fuel tank pressure sensor measures the pressure changes within the EVAP system. The fuel tank pressure sensor has a 5.0 volt reference circuit, a ground circuit, and a signal circuit.

The fuel tank pressure sensor contains a diaphragm that changes resistance based on pressure. When the EVAP system pressure is low (during purge), the sensor output voltage is low. When the system pressure is high, the sensor output voltage is high. The signal from this sensor provides feedback to the ECM on the EVAP system operation. The ECM's programming contains information on expected EVAP system behavior under various operating conditions. The ECM monitors the EVAP system pressure. If the fuel tank pressure sensor signal is outside of the normal operating range, then DTC P0450 Fuel Tank Pressure Sensor Circuit will set.

If the fuel tank pressure sensor signal differs from an expected result during an EVAP system test (caused by either a sensor problem or an actual EVAP system problem), the following DTCs can set:

The wide range heated oxygen sensor (HO2S) calculates the amount of oxygen in the exhaust stream more accurately than the toggling style HO2S. The wide range HO2S sensor acts more like an air fuel sensor. The engine control module (ECM) supplies a voltage (approx. 28 volt) to the HO2S and uses this voltage as a reference to the amount of oxygen in the exhaust system. When the system is lean, the oxygen level will be high and the reference voltage also will be high. When the oxygen level is low, the reference voltage will also be low. The ECM monitors the variation in voltage and attempts to keep the voltage constant by increasing or decreasing the amount of current flow to the HO2S. Using this information allows the ECM to maintain the proper air/fuel ratio.

A three-way catalytic converter is used to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction that oxidizes the HC and the CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The ECM monitors this process by using the Bank 1 HO2S 2 and the Bank 2 HO2S 2 heated oxygen sensors. The front HO2S sensors produce an output signal that indicates how much oxygen is present in the exhaust gas entering the three-way catalytic converter. The rear HO2S sensors produce an output signal that indicates the oxygen storage capacity of the catalyst. The catalyst's oxygen storage capacity indicates the catalyst's ability to convert the exhaust gases efficiently.

The Intake Air Temperature (IAT) sensor contains a semiconductor device that changes resistance based on temperature (a thermistor). The IAT sensor is located within the mass air flow sensor. The IAT sensor has a signal circuit and a ground circuit. The ECM applies a voltage of about 5.0 volts on the signal circuit to the sensor. The ECM monitors changes in this voltage caused by changes in the resistance of the sensor in order to determine the intake air temperature.

When the intake air is cold, the sensor (thermistor) resistance is high, and the ECM's signal voltage is only pulled down a small amount through the sensor to ground. Therefore, the ECM will sense a high signal voltage (low temperature). When the intake air is warm, the sensor resistance is low, and the signal voltage is pulled down a greater amount. This causes the ECM to sense a low signal voltage (high temperature).

The scan tool displays the temperature of the air entering the engine. This reading should be close to the ambient air temperature when the engine is cold. The temperature reading should rise as the underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT sensor temperature and the engine coolant temperature should be similar.

(1)	Electrical Connector
(2)	MAF Sensor
(3)	Circuit Board Cover
(4)	Circuit Board
(5)	IAT Sensor
(6)	Circuitry Housing

The mass air flow (MAF) sensor measures the amount of air coming into the engine. This direct airflow measurement is more accurate than the calculated airflow information obtained from the other sensor inputs. The MAF sensor has a switched battery feed, a ground, a signal circuit, and a signal return circuit.

The MAF sensor that is used on this vehicle is a hot film type and is used in order to measure the air flow rate. The MAF output voltage is a function of the power required to keep the air flow sensing elements at a fixed temperature above the ambient temperature. The air flowing through the sensor cools the sensing elements. The amount of cooling is proportional to the amount of air flow. As the air flow increases, more current is needed in order to maintain the hot film at a constant temperature. The MAF sensor converts the changes in the current draw to a voltage signal that is read by the ECM. The ECM calculates the air flow based on this signal.

The ECM monitors the MAF sensor signal voltage and can determine if the sensor signal voltage is too low or too high. The ECM can also detect airflow that is inappropriate for a given operating condition based on the signal voltage, or a signal that appears to be stuck based on the lack of normal signal fluctuations expected during engine operation.

The scan tool reads the MAF value and displays the value in grams per second (gm/s). Values should change rather quickly on acceleration, but should remain fairly stable at any given RPM. When the ECM detects a malfunction in the MAF sensor or circuit, DTC P0100 Mass Air Flow (MAF) Sensor Circuit will set:

Theft Deterrent Circuits

The ECM and the theft deterrent module exchange data using two existing circuits. The MIL Control circuit connects to the ECM and the theft deterrent module. When the ignition switch is turned on, and the ECM illuminates the MIL (bulb check), the voltage change on the MIL Control circuit is used as a wake up signal to the theft deterrent module. If a problem with this circuit prevents the theft deterrent module from receiving the wake up signal, a DTC will set. The vehicle speed signal circuit is also connected to both the ECM and the theft deterrent module. The theft deterrent system frequency code relays between the two controllers via this circuit. The DTCs that apply to problems involving the exchange of data between the ECM and the theft deterrent module are as follows:

•	DTC P1501 Theft Deterrent System

•	DTC P1502 Theft Deterrent Fuel Enable Signal Not Received

•	DTC P1503 Theft Deterrent Fuel Enable Signal Not Correct

Electronic Throttle Control

The throttle actuator control (TAC) system is used to improve emissions, fuel economy, and driveability. The TAC system replaces the mechanical link between the accelerator pedal and the throttle valve. The TAC systems also eliminate the idle air controller (IAC) valve, and the cruise control module. These systems are now controlled by the Throttle Actuator Control (TAC) module (integrated into the ECM). The TAC system contains the following components:

•	The accelerator pedal position (APP) sensor

•	The throttle position (TP) sensor

•	The throttle body assembly

•	Throttle actuator motor

•	The engine control module (ECM)

Each of these components will be described briefly in the following paragraphs:

Accelerator Pedal Position (APP) Sensor

The APP module is mounted on the accelerator pedal assembly. The APP is actually 2 individual accelerator pedal position sensors within one housing. Two separate signal, ground, and 5 volt reference circuits are used to connect the APP to the engine control module. The APP sensor 1 voltage should increase as the accelerator pedal is depressed, from below -3 volts at 0 percent pedal travel to above 4 volts at 100 percent pedal travel. The APP sensor 2 should increase from about -3 volts at 0 throttle to 2-1 at 100 percent travel.

Throttle Position Sensors

The throttle body for the ETC system is similar to a conventional throttle body with a couple of exceptions. One exception is the use of a motor to control the throttle position instead of a mechanical cable. The other exception is the newly designed throttle position sensor. The TP sensor mounts on the side of the throttle body opposite the throttle actuator motor. The TP sensor consists of one shared 5 volt reference circuit, a shared ground circuit, and 2 signal circuits. The TP sensor must be replaced with the throttle body assembly. The TP sensor 1 signal voltage increases as the throttle opens, from approximately 1.0 volts at 0 percent throttle to more than 3.4 volts at 100 percent throttle. TP sensor 2 signal voltage decreases as the throttle is opened, from more than 3.9 volts at 0 percent throttle to less than 1.5 volts at 100 percent throttle.

Throttle Body Assembly

The throttle body assembly consists of the throttle body, the TP sensors, and the throttle drive motor. None of these components are serviced individually. They must be serviced as an assembly. The throttle body still functions the same as in the past, except that an electronic motor now opens and closes the throttle valve.

Throttle Actuator Motor

The throttle actuator motor moves the throttle plates when commanded by the ECM. The throttle actuator motor is not serviceable separately and is replaced with the throttle body assembly.

Electronic Control Module (ECM)

The TAC module is the control center for the electronic throttle system. The TAC module is located inside the ECM and is not serviceable separately. The TAC module and the ECM monitor the commanded throttle position and compare it to the actual throttle position. This is accomplished by monitoring the APP and the TP sensors. These two values must be within a calibrated value of each other. The ECM module also monitors each individual circuit of the TP sensor and the APP to verify proper operation.

Idle Learn Procedure

Whenever the throttle body assembly or the engine control module (ECM) is replaced, you must perform the idle learn procedure. To perform this procedure, turn ON the ignition while leaving the engine OFF for 30 seconds. Key down, then restart the vehicle, and the normal idle should return.

Vehicle Speed Signal Circuit

The ECM receives the Vehicle Speed data from the Antilock Brake System (ABS). The vehicle speed is calculated by the ABS controller from the wheel speed sensor signals, and is sent to the ECM via the Vehicle Speed Signal circuit. If the ECM detects a problem with this circuit, DTC P0500 Vehicle Speed Sensor (VSS) Circuit will set.