For 1990-2009 cars

All of the sensors and the input switches can be diagnosed through the use of a scan tool. The following is a short description of how the sensors and switches can be diagnosed by using a scan tool. The scan tool can also be used in order to compare the values for a normal running engine with the engine being diagnosed.

### Engine Coolant Temperature (ECT) Sensor

(1) | ECT Electrical Connector
(2) | Connector Tab
(3) | Engine Coolant Temperature (ECT) Sensor

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant pump. Low coolant temperature produces a high resistance (100,000 ohms at -40°C/-40°F) while high temperature causes low resistance (70 ohms at 130°C/266°F).

The PCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the engine is cold. The voltage will be low when the engine is hot. By measuring the voltage, the PCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the PCM controls.

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 PCM detects a malfunction in the ECT sensor circuit, the following DTCs will set:

• | DTC P0117 circuit low.

• | DTC P0118 circuit high.

• | DTC P0125 excessive time to Closed Loop.

• | DTC P1114 circuit intermittent low.

• | DTC P1115 circuit intermittent high.

Service Category Specifications contains a table to check for sensor resistance values relative to temperature.

### Mass Air Flow (MAF) Sensor

The Mass Air Flow (MAF) sensor measures the amount of air which passes through it. The PCM uses this information to determine the operating condition of the engine, to control fuel delivery. A large quantity of air indicates acceleration. A small quantity of air indicates deceleration or idle.

The scan tool reads the MAF value and displays it in grams per second (gm/Sec). At idle, it should read between 6 gm/Sec - 9 gm/Sec on a fully warmed up engine. Values should change rather quickly on acceleration, but values should remain fairly stable at any given RPM. When the PCM detects a malfunction in the MAF sensor circuit, the following DTCs will set:

• | DTC P0100 circuit malfunction.

• | DTC P0101 system performance.

• | DTC P0102 frequency low.

• | DTC P0103 frequency high.

### Intake Air Temperature (IAT) Sensor

(1) | Intake Air Temperature (IAT) Sensor
(2) | Electrical Harness Connector

The Intake Air Temperature (IAT) sensor is a thermistor which changes value based on the temperature of air entering the engine. Low temperature produces a high resistance (100,000 ohms at -40°C/-40°F). A high temperature causes low resistance (70 ohms at 130°C/266°F). The PCM supplies a 5.0 volt signal to the sensor through a resistor in the PCM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the PCM calculates the incoming air temperature. The IAT sensor signal is used to adjust spark timing according to incoming air density.

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 PCM detects a malfunction in the IAT sensor circuit, the following DTCs will set:

• | DTC P0112 circuit low.

• | DTC P0113 circuit high.

• | DTC P1111 circuit intermittent high.

• | DTC P1112 circuit intermittent low.

### Manifold Absolute Pressure (MAP) Sensor

(1) | Electrical Connector
(2) | Manifold Absolute Pressure (MAP) Sensor

The Manifold Absolute Pressure (MAP) sensor responds to changes in the intake manifold pressure. The pressure changes as a result of engine load and speed. The map sensor converts this to a voltage output.

A closed throttle on engine coast down would produce a relatively low map output voltage. A wide open throttle would produce a high map output voltage. This high output voltage is produced because the pressure inside the manifold is the same as outside the manifold. The MAP is inversely proportional to what is measured on a vacuum gage. The MAP sensor is used for the following:

• | Altitude determination.

• | Ignition timing control.

• | EGR diagnostic.

• | Speed density fuel management default.

When the PCM detects a malfunction in the MAP sensor circuit, the following DTCs will set:

• | DTC P0106 circuit performance malfunction.

• | DTC P0107 circuit low.

• | DTC P0108 circuit high.

• | DTC P1107 intermittent circuit low.

### Heated Oxygen Sensors (HO2S) Cutaway

(1) | Four Wire In-Line Connector
(2) | Heater Termination
(3) | Water Shield Assembly
(4) | Sensor Lead
(5) | Flat Seat Shell
(6) | Seat Gasket
(7) | Outer Electrode and Protective Coating
(8) | Rod Heater
(9) | Inner Electrode
(10) | Zirconia Element
(11) | Insulator
(12) | Clip Ring
(13) | Gripper

### Front Heated Oxygen Sensors (HO2S)

The heated oxygen sensors (HO2S) are mounted in the exhaust system where they can monitor the oxygen content of the exhaust gas stream. The oxygen present in the exhaust gas reacts with the sensor to produce a voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates what fuel mixture command to give to the injectors (lean mixture - low HO2S voltage = rich command, rich mixture - high HO2S voltage = lean command).

When the PCM detects an HO2S signal circuit that is low, the PCM will set the following DTCs:

• | DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1

• | DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1

• | DTC P0137 HO2S Circuit Low Voltage Bank 1 Sensor 2

• | DTC P0157 HO2S Circuit Low Voltage Bank 2 Sensor 2

When the PCM detects an HO2S signal circuit that is high, the PCM will set the following DTCs:

• | DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1

• | DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1

• | DTC P0138 HO2S Circuit High Voltage Bank 1 Sensor 2

• | DTC P0158 HO2S Circuit High Voltage Bank 2 Sensor 2

When the PCM detects no HO2S activity, the PCM will set the following DTCs:

• | DTC P0134 HO2S Insufficient Activity Bank 1 Sensor 1

• | DTC P0154 HO2S Insufficient Activity Bank 2 Sensor 1

• | DTC P0140 HO2S Insufficient Activity Bank 1 Sensor 2

• | DTC P0160 HO2S Insufficient Activity Bank 2 Sensor 2

A fault in the heated oxygen sensor heater element or its ignition feed or ground will result in an increase in time to Closed Loop fuel control. This may cause increased emissions, especially at start-up. When the PCM detects a malfunction in the HO2S heater circuits, the following DTCs will set:

• | DTC P0135 Heater Circuit Bank 1 Sensor 1

• | DTC P0155 Heater Circuit Bank 2 Sensor 1

• | DTC P0141 Heater Circuit Bank 1 Sensor 2

• | DTC P0161 Heater Circuit Bank 2 Sensor 2

The PCM also has the ability to detect the following HO2S problems:

• | HO2S response

• | Switching

• | Transition time

• | Incorrect ratio voltage

The PCM stores a DTC that indicates degraded HO2S performance if any of the above is detected.

### Rear Heated Oxygen Sensors (HO2S)

To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The PCM has the ability to monitor this process using the Bank 1 HO2S 2 and the Bank 2 HO2S 2 heated oxygen sensors. The front HO2S sensors produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the three-way catalytic converter. The rear HO2S sensors produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalysts ability to convert 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 catalyst monitor sensors operate the same as the fuel control sensors. Although the Bank 1 HO2S 2 and Bank 2 HO2S 2 sensors main function is catalyst monitoring, they also play a limited role in fuel control. If a sensor output indicates a voltage either above or below the 450 millivolt bias voltage for an extended period of time, the PCM will make a slight adjustment to fuel trim to ensure that fuel delivery is correct for catalyst monitoring

### Throttle Position (TP) Sensor

(1) | Powertrain Control Module (PCM)
(2) | Throttle Position (TP) Sensor
(3) | Throttle Valve

The Throttle Position (TP) sensor is a potentiometer. The TP sensor is connected to the throttle shaft on the throttle body. By monitoring the voltage on the signal line, the PCM calculates throttle position. As the throttle valve angle is changed (accelerator pedal moved), the TP sensor signal also changes. At a closed throttle position, the output of the TP sensor is low. As the throttle valve opens, the output increases so that at Wide Open Throttle (WOT), the output voltage should be above 4.0 volts.

The PCM calculates fuel delivery based on throttle valve 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 PCM detects the throttle is moving.

When the PCM detects a malfunction with the TP sensor circuits, the following DTCs will set:

• | DTC P0121 circuit performance malfunction.

• | DTC P0122 circuit low.

• | DTC P0123 circuit high.

• | DTC P1121 intermittent circuit high.

• | DTC P1122 intermittent circuit low.

### EVAP Purge Vacuum Switch

The EVAP Purge Vacuum Switch is used by the PCM to monitor EVAP canister purge solenoid operation and purge system integrity. The EVAP Purge Vacuum Switch should be closed to ground with no vacuum present (0% EVAP Purge PWM). With EVAP Purge PWM at 25% or greater, the EVAP Purge Vacuum Switch should open.

An incorrect EVAP Purge system flow should set a DTC P0441. A continuous purge condition with no purge commanded by the PCM should set a DTC P1441. Refer to Evaporative Emission (EVAP) Control System for a complete description of the EVAP system.

### Knock Sensors (KS)

The Knock Sensor (KS) system is used to detect engine detonation. The PCM will retard the spark timing based on the signals from the KS module. The Knock Sensor(s) produce an AC voltage that is sent to the KS module. The amount of AC voltage produced is proportional to the amount of knock.

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

### A/C Request Signal

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

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

### A/C Refrigerant Pressure Sensor

This signal is used by the PCM to enable the cooling fans when head pressure reaches a predetermined amount. If a fault is present in the A/C refrigerant pressure sensor circuit, DTC P0530 or P0531 will set. When the PCM sets a DTC, the A/C compressor clutch will be disabled.

### A/C Evaporator Temperature Sensor

This signal is used by the PCM to cycle the A/C clutch ON and OFF. The PCM will cycle the clutch ON at 4°C (39°F) and OFF at 2°C (36°F). A failure in the A/C evaporator temperature sensor circuit will result in a DTC P1532 and P1533. When an A/C evaporator temperature sensor DTC sets, the A/C clutch will be disabled.

### Park/Neutral Position (PNP) Switch (Automatic Transmission)

The PNP switch indicates to the PCM when the transmission is in park, neutral, or drive. This information is used for the EGR and IAC valve operation. Refer to Electrical Diagnosis for more information on the PNP switch. The PNP switch is part of the neutral/start and backup light switch assembly. Refer to Park/Neutral Position Switch diagnosis.

Important:: Vehicle should not be driven with the PNP switch disconnected, as idle quality will be affected. Having the switch disconnected may also cause a VSS DTC to set.

### Vehicle Speed Sensor (VSS)

The Vehicle Speed Sensor (VSS) is a pulse counter type input that informs the PCM how fast the vehicle is being driven. The VSS system uses an inductive sensor mounted in the tail housing of the transmission and a toothed reluctor wheel on the tail shaft. As the reluctor rotates, the teeth alternately interfere with the magnetic field of the sensor creating an induced voltage pulse.

The VSS produces an AC voltage signal that increases with vehicle speed. The PCM processes this signal and sends it to the following components:

• | Instrument Panel.

• | Radio control head.

• | Chime Module.

• | Cruise Control Module.

### Crankshaft Position Sensor (CKP)

The crankshaft position sensor provides the PCM with crankshaft speed and crankshaft position. The PCM utilizes this information to determine if an engine Misfire is present. The PCM monitors the CKP sensor for momentarily drop in crankshaft speed to determine if a misfire is occurring. When the PCM detects a misfire, a DTC P0300 will set.

The PCM also monitors the CKP sensor signal circuit for malfunctions. The PCM monitors CKP signal and the High and Low resolution signals. The PCM calculates these signals to determine a ratio. When the PCM detects that the ratio is out of normal operating range, the PCM will set a DTC P0335 or a DTC P0336.