Information Sensors/Switches Description. Pontiac Firebird 1999

For 1990-2009 cars only

Makes 🢒 Pontiac 🢒 1999 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 Information Sensors/Switches Description

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 left cylinder head. Low coolant temperature produces a high resistance, 100,000 ohms at -38°C (-39°F), while high temperature causes low resistance, 70 ohms at 130°C (266°F).

The PCM supplies a 5.0 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. The PCM calculates the engine coolant temperature by measuring the voltage. The engine coolant temperature affects most systems the PCM controls.

The scan tool displays engine coolant temperature in degrees. When the engine starts the engine coolant temperature should rise steadily to about 90°C (194°F) then stabilize when the thermostat opens. The engine coolant temperature and intake air temperature parameter displays should be close to each other if the engine has not been run for several hours (overnight). The following DTCs set when the PCM detects a malfunction in the ECT sensor circuit:

•	DTC P0117 ECT Sensor Circuit Low Voltage.

•	DTC P0118 ECT Sensor Circuit High Voltage.

•	DTC P0125 Excessive Time to Closed Loop.

•	DTC P1114 ECT Sensor CKT Intermittent Low Voltage.

•	DTC P1115 ECT Sensor CKT Intermittent High Voltage.

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 in order to control the 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 (g/s). The MAF sensor displays between 6 - 9 g/s at idle on a fully warmed up engine. The MAF sensor values should change rather quickly on acceleration, but the MAF sensor values should remain fairly stable at any given engine speed. One or more of the following DTCs set when the PCM detects a malfunction in the MAF sensor circuit:

•	DTC P0101 Mass Air Flow System Performance.

•	DTC P0102 MAF Sensor Circuit Low Frequency.

•	DTC P0103 MAF Sensor Circuit High Frequency.

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 -38°C (-39°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. The PCM calculates the incoming air temperature by measuring the IAT voltage. The IAT sensor signal is used to adjust spark timing according to incoming air density.

The scan tool displays the temperature of the air entering the engine, which should be close to ambient air temperature when the engine is cold. The temperature should rise as underhood temperature increases. The engine coolant temperature and intake air temperature parameter displays should be close to each other if the engine has not been run for several hours (overnight).

The following DTCs set when the PCM detects a malfunction in the IAT sensor circuit:

•	DTC P0112 IAT Sensor Circuit Low Voltage.

•	DTC P0113 IAT Sensor Circuit High Voltage.

•	DTC P1111 IAT Sensor Circuit Intermittent High Voltage.

•	DTC P1112 IAT Sensor Circuit Intermittent Low Voltage.

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

Manifold Absolute Pressure (MAP) Sensor

The Manifold Absolute Pressure (MAP) sensor (1) 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 produces a relatively low MAP output voltage. A wide open throttle produces 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 gauge. The MAP sensor is used for the following:

•	Altitude determination.

•	Ignition timing control.

•	Speed density fuel management default.

DTC P0107 MAP Sensor Circuit Low Voltage or DTC P0108 MAP Sensor Circuit High Voltage set when the PCM detects a malfunction in the MAP sensor circuit.

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 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 the fuel pulse width command to give to the injectors (lean mixture/low HO2S voltage = rich command, rich mixture/high HO2S voltage = lean command).

The PCM sets the following DTCs when the PCM detects an HO2S signal circuit that is low:

•	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.

The PCM sets the following DTCs when the PCM detects an HO2S signal circuit that is high:

•	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.

The PCM sets the following DTCs when the PCM detects no HO2S activity:

•	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 results in an increase in time to Closed Loop fuel control. This may cause increased emissions, especially at start-up. The following DTCs set when the PCM detects a malfunction in the HO2S heater circuits:

•	DTC P0135 HO2S Heater Circuit Bank 1 Sensor 1.

•	DTC P0155 HO2S Heater Circuit Bank 2 Sensor 1.

•	DTC P0141 HO2S Heater Circuit Bank 1 Sensor 2.

•	DTC P0161 HO2S 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 voltages

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

The PCM performs the catalyst diagnostic at idle when the conditions for running the diagnostic are met. During the catalyst diagnostic the PCM captures the current rear HO2S rich/lean status. The Air Fuel ratio transitions from rich to lean or lean to rich depending on the initial captured rich/lean status. The Air Fuel ratio transitions a second time opposite the first Air Fuel ratio transition. During this diagnostic the scan tool will display HO2S voltages going from full rich to full lean. This condition is normal during this diagnostic.

Throttle Position (TP) Sensor

The Throttle Position (TP) sensor (2) is a potentiometer. The TP sensor is connected to the throttle shaft (3) on the throttle body. The PCM (1) calculates throttle position by monitoring the voltage on the signal line. The TP sensor signal changes as the throttle valve angle is changed (accelerator pedal moved). The TP sensor signal voltage is low at a closed throttle position. The TP sensor signal voltage increases as the throttle valve opens 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.

The following DTCs set when the PCM detects a malfunction with the TP sensor circuits:

•	DTC P0121 TP Sensor Circuit Insufficient Activity.

•	DTC P0122 TP Sensor Circuit Low Voltage.

•	DTC P0123 TP Sensor Circuit High Voltage.

•	DTC P1121 TP Sensor CKT Intermittent High Voltage.

•	DTC P1122 TP Sensor CKT Intermittent Low Voltage.

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. The PNP switch is part of the neutral/start and backup light switch assembly.

Important:: Idle quality will be affected if the vehicle is driven with the PNP switch disconnected. Having the switch disconnected may also cause a VSS DTC to set.

Fuel Level Sensor


(1)	Fuel Pressure Sensor
(2)	Fuel Pressure Regulator Retaining Clip
(3)	Fuel Pressure Regulator
(4)	Fuel Strainer
(5)	Fuel Level Sensor
(6)	Fuel Pump Electrical Connector Retaining Clip
(7)	Fuel Pump Electrical Connector
(8)	Fuel Level Sensor Electrical Connector
(9)	Fuel Level Sensor Electrical Connector Retaining Clip

The PCM uses the fuel level sensor input in order to determine the amount of fuel in the fuel tank. The PCM disables the engine misfire diagnostic when the fuel level is too low and the EVAP system diagnostic when the fuel level is either too high or too low. The PCM also controls the fuel gauge based on the fuel level input.

Fuel Tank Pressure Sensor

The Fuel Tank Pressure sensor is a sensor much like the MAP sensor. The Fuel Tank Pressure sensor measures the difference between the outside air pressure and the air pressure (or vacuum) in the fuel tank.

The Fuel Tank Pressure sensor mounts to the fuel tank sending unit. A three wire electrical harness connects the sensor to the PCM. The PCM supplies a 5.0 volt reference voltage and a ground to the sensor. The sensor will return a voltage between 0.1 and 4.9 volts on the signal circuit.

Knock Sensors (KS)

A Knock Sensor (KS) system is used in order to control spark knock. The KS system is designed to retard spark timing up to 20 degrees in order to reduce spark knock in the engine. This allows the engine to use maximum spark advance to improve driveability and fuel economy.

The knock sensor system is used to detect engine detonation. The knock sensors produce an AC voltage which is sent to the PCM. The PCM retards the spark timing based on signals from the KS sensors. The amount of AC voltage produced by the sensors is determined by the amount of knock. The PCM then adjusts the Ignition Control (IC) to reduce the spark knock.

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 in order to enable the A/C compressor clutch and to adjust the idle speed before turning ON the A/C clutch. The PCM disables the A/C compressor clutch if the A/C request signal is not available to the PCM.

A/C Refrigerant Pressure Sensor

This signal is used by the PCM in order to control the cycling of the compressor. It will also enable the cooling fans when the A/C compressor head pressure reaches a predetermined value. DTC P0530 sets if a fault is present in the A/C refrigerant pressure sensor circuit. The PCM disables the A/C compressor clutch when the PCM sets an A/C system DTC.

Vehicle Speed Sensor (VSS)

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

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 in order to determine if an engine misfire is present. The PCM monitors the CKP sensor for momentarily drop in crankshaft speed in order to determine if a misfire is occurring. A DTC P0300 sets when the PCM detects a misfire.

The PCM also monitors the CKP sensor signal circuit for malfunctions. The PCM sets a DTC P0335 or a DTC P0336 when the CKP sensor is out of the normal operating range.

Camshaft Position Sensor (CMP)

The Camshaft Position sensor is mounted through the top of the engine block at the rear of the valley cover and works in conjunction with a 1X reluctor wheel on the camshaft. The reluctor wheel is inside the engine immediately in front of the rear cam bearing. The PCM provides a 12 volt power supply to the CMP sensor as well as a ground and a signal circuit.

The PCM uses the Camshaft Position sensor in order to determine whether a cylinder is on a firing or exhaust stroke. The reluctor wheel interrupts a magnetic field produced by a magnet within the CMP sensor as the camshaft rotates. The CMP sensor's internal circuitry detects this and produces a signal which is the PCM reads. The PCM uses this 1X signal in combination with the Crankshaft Position sensor 24X signal in order to determine the crankshaft position and stroke. This diagnostic for the Camshaft Position sensor checks for a loss of Camshaft Position sensor signal. The PCM also monitors the CMP sensor signal circuit for malfunctions. The following DTCs set when the PCM detects a CMP sensor that is out of the normal operating range.

•	DTC P0341 Camshaft Position Sensor (CMP) Circuit Performance.

•	DTC P0342 Camshaft Position Sensor (CMP) Circuit Low Voltage.

•	DTC P0343 Camshaft Position Sensor (CMP) Circuit High Voltage.