You can diagnose all of the sensors and most of the input circuits with a scan tool. In 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 a normal running engine 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 is used in order to detect an engine detonation. 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 that is produced is proportional to the amount of knock.

An operating engine produces a normal amount of an engine mechanical vibration (Noise). The knock sensors will produce an AC voltage signal from this Noise. When an engine is operating, the ECM will learn the minimum and the maximum frequency of the noise that the engine produces. When the ECM determines that this frequency is less than or greater than the expected amount, a knock sensor DTC will set.

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 Crankshaft Position (CKP) sensor works in conjunction with a 58 tooth reluctor wheel on the crankshaft. The ECM pulls up the CKP sensor signal circuit to 12 volts and monitors this voltage. As each 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 teeth are 6 degrees apart. Having only 58 teeth leaves a 12 degree span that is uncut. This creates a signature pattern that enables the ECM in order to determine the crankshaft position. The ECM can determine which two cylinders are approaching the top dead center based on the CKP signal alone. The Camshaft Position (CMP) sensor signal is used in order 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 for information on misfire detection.

The ECM also monitors the CKP sensor system for malfunctions. The following DTCs indicate that the ECM has detected a CKP system problem:

The Engine Coolant Temperature (ECT) sensor (1) contains a semiconductor device which changes the 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 (about 5.0 volts) on the signal circuit to the sensor. The ECM monitors changes in this voltage that are 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. Therefore, the ECM will sense a high signal voltage (low temperature). When the engine coolant 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 engine coolant temperature in degrees. After engine startup, the temperature should rise steadily to about 90°C (194°F) then stabilize when 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 circuit, the following DTC(s) will set:

The Service Category Specifications contain a table in order to check for sensor resistance values relative to the temperature. Refer to Temperature vs Resistance .

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, a ground and a signal circuit.

The Fuel Tank Pressure sensor contains a diaphragm which changes the resistance based on the 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 an expected system behavior during the various EVAP system operating conditions. The ECM monitors the EVAP system pressure. If the Fuel Tank Pressure sensor signal is outside of the normal operating range of the sensor, the 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 Heated Oxygen Sensor (HO2S) produces a voltage that varies between 100 mV and 900 mV under the normal operating conditions. On the Catera, the full operating range of the HO2S circuit is approximately from -200 mV to 1,200 mV. The Engine Control Module (ECM) monitors this voltage and determines if the exhaust is lean or rich. The oxygen sensor voltage is high when the exhaust is rich, and low when the exhaust is lean. The ECM constantly monitors the HO2S signal during the closed loop operation and compensates for a rich or lean condition by decreasing or increasing the injector pulse width as necessary.

The following DTCs indicate that the ECM has detected an HO2S signal voltage that is outside of the normal operating range of the sensor:

The following DTCs indicate that the ECM has detected an HO2S signal voltage that remains excessively low:

The following DTCs indicate that the ECM has detected an HO2S signal voltage that remains excessively high:

A fault in the heated oxygen sensor heater element or the related ignition feed or ground will result in an increase in time to a Closed Loop fuel control. This may cause increased emissions, especially at start-up. The following DTCs indicate that the ECM detects a problem with an HO2S Heater:

The ECM also has the ability to evaluate HO2S response time and whether or not there is sufficient signal activity. If a problem is detected, the ECM will store a DTC indicating degraded HO2S performance.

Use a three-way catalytic converter in order to control the emissions of the Hydrocarbons (HC), the Carbon Monoxide (CO), and the Oxides of Nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and the CO that is present in the exhaust gas, converting the HC and the CO into a harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting the NOx to nitrogen. The ECM has the ability to monitor 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 which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The rear HO2S sensors produce an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert the exhaust gases efficiently. If the catalyst is operating efficiently, the front sensors will produce a far more active signal than that produced by the rear sensors.

The Intake Air Temperature (IAT) sensor [grommet (1)] contains a semiconductor device which changes the resistance based on temperature (a thermistor). The IAT sensor is in the air intake passage of the engine air induction system. The IAT sensor has a signal circuit and a ground circuit. The ECM applies a voltage (about 5.0 volts) on the signal circuit to the sensor. The ECM monitors changes in this voltage that are caused by the 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 temperature of the air entering the engine, which should read close to ambient air temperature when engine is cold. The temperature should rise as underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT sensor temperature and engine coolant temperature should read close to each other. If the ECM detects a malfunction in the IAT sensor circuit, the DTC P0110 -- IAT Sensor Circuit will set.

The Mass Air Flow (MAF) sensor measures the amount of air that is ingested by the engine. The direct measurement of the air entering the engine is more accurate than the calculating airflow 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 that is required in order 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, a greater amount of current is required 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, too high, not indicating the expected airflow for a given operating condition, or that the signal appears to be stuck based on the lack of normal signal fluctuations that are expected during the 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 circuit, the following DTC(s) will set:

This DTC indicates that a transmission related OBD II failure has occurred. The Transmission Control Module (TCM) controls a dedicated Service Transmission Lamp (STL) which illuminates when a fail is reported by certain non-emissions related TCM diagnostics. The TCM has no direct control of the engine MIL, but if a transmission fault occurs that is emissions related, the engine MIL must illuminate. A MIL Request circuit between the ECM and the TCM provides a means for illuminating the MIL, even though the fault was detected by the TCM. This circuit is pulled up to B+ within the ECM. In order to illuminate the MIL, the TCM pulls the circuit low. The ECM detects this low circuit and the DTC P1700 sets, which illuminates the MIL. If the ECM detects a problem with the MIL Request circuit, the DTC P1701 -- MIL Request circuit will set.

The TCM pulls this circuit up to 12 volts in gear positions R, D, 3, 2, 1. The circuit voltage is pulled low in P/N. The ECM monitors the circuit voltage in order to compensate for in-gear engine load. If the TCM detects a problem on this circuit, the following DTC P1705 - Park/Neutral Position Signal Circuit will set.

Refer to Transmission for diagnosis of Transmission DTCs

The ECM pulls this circuit up to 5 volts and monitors the circuit voltage. The ABS Controller pulls the circuit voltage down in proportion to the severity of the rough road detected. The ECM needs to comprehend the rough road data in the calculations used in order to detect the engine misfire. Refer to Rough Road Detection Signal Circuit diagnosis.

There are no circuits dedicated to communication between the ECM and the Theft Deterrent Module. 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. DTCs that apply to the problems that are detected with the exchange of data between the ECM and the Theft Deterrent Module are as follows:

The Throttle Position (TP) sensor is a potentiometer. The TP sensor (2) [electrical connector (1)] mounts on the side of the throttle body. The TP sensor provides a voltage signal that changes relative to the throttle blade angle. This signal voltage is one of the most important inputs that is used by the ECM. The TP sensor has a 5.0 volt reference, a ground and a signal circuit. The signal circuit is pulled up to 5 volts within the ECM.

The ECM calculates the fuel delivery based on the throttle angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector. This may cause an unstable idle because the ECM detects the throttle is moving. When the ECM detects a malfunction with the TP sensor circuit, the DTC P0120 -- TP Sensor Circuit will set.

The ECM pulls this circuit up to 12 volts and monitors this circuit. The TCM also monitors this circuit. This circuit provides bi-directional communication between the two controllers. Messages are sent every 25 msec. This data is used in order to reduce the engine torque during transmission shift events in order to improve the shift feel. If the ECM detects a problem on this circuit, the DTC P1740 -- Torque Control Circuit will set.

The Antilock Brake System controller sends a signal to the ECM indicating that an excessive wheel spin has been detected. This data is used in order to reduce the engine torque in order to reduce the wheel spin and improve the traction. If the TCM detects a problem on this circuit, the DTC P1844 -- Traction Control Torque Reduction Request Circuit will set.

Refer to Transmission for diagnosis of the Transmission DTCs

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, the DTC P0501 -- Vehicle Speed Signal Circuit will set.