Intermittent Failures
The term "intermittent failure" means a system may have had a failure, but it checks OK now. If the Malfunction Indicator Lamp (MIL) on the dash does not come on, check for poor connections or loose terminals at all connectors related to the circuit that you are troubleshooting.
Opens & Shorts
"Open" and "Short" are common electrical terms. An open is a break in a wire or at a connection. A short is an accidental connection of a wire to ground or to another wire. In simple electronics, this usually means something won't work at all. With complex electronics such as PCMs, this can sometimes mean something works, but not the way it's supposed to.
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- Start the engine, and check the MIL (A). NOTE: If the ignition switch is turned to ON (II), and the engine is not started, the MIL will stay on for 15-20 seconds (see «Malfunction Indicator Lamp (MIL) Indication (In relation To Readiness Codes)»(ref-173995-S12454021692005041500000) ).
- If the MIL stays on, connect the HDS or the scan tool to the Data Link Connector (DLC) (A) located behind driver's dashboard lower cover.
- Turn the ignition switch ON (II).
- Check the diagnostic trouble code (DTC) and note it. Also check the freeze data. Refer to the DTC Troubleshooting Index, and begin the appropriate troubleshooting procedure.
Note. Freeze data indicates the engine conditions when the first malfunction, misfire, or fuel trim malfunction was detected. The HDS and the scan tool can read the DTC, freeze data, current data, and other powertrain control module (PCM) data. For specific operations, refer to the user's manual that came with the HDS or the scan tool.
If The MIL Did Not Stay On
If the MIL did not stay on, but there is a driveability problem, do the symptom troubleshooting.
Electronic Control System
The functions of the fuel and emission control systems are managed by the powertrain control module (PCM).
Fail-Safe Function
When an abnormality occurs in the signal from a sensor, the PCM ignores that signal and assumes a pre-programmed value for that sensor that allows the engine to continue to run.
Back-Up Function
When an abnormality occurs in the PCM, the injectors are controlled by a back-up circuit independent of the system in order to permit minimal driving.
Self-Diagnosis
When an abnormality occurs in the signal from a sensor, the PCM supplies ground for the Malfunction Indicator Lamp (MIL) and stores the diagnostic trouble code (DTC) in erasable memory. When the ignition is first turned on, the PCM supplies ground to the MIL for 15 to 20 seconds to check the MIL bulb condition.
Two Driving Cycle Detection Method
To prevent false indications, the "two driving cycle detection method" is used for some self-diagnostic functions. When an abnormally occurs, the PCM stores it in its memory. When the same abnormality recurs after the ignition switch is turned OFF and ON (II) again, the PCM turns on the MIL.
PCM Data
You can retrieve data from the PCM by connecting the HDS or the scan tool to the data link connector (DLC). The items listed in the table below conform to SAE recommended practice. The HDS also reads data beyond that recommended by SAE to help you find the causes of intermittent problems.
Note. The "operating values" listed are approximate and may vary depending on the environment and the individual vehicle. Unless noted otherwise, "at idle speed" means idling with the engine completely warmed up, A/T in the Park or the neutral position, and the A/C and all accessories turned off.
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Note. Standard battery voltage is 12 V.
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Note. Standard battery voltage is 12 V.
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Note. Standard battery voltage is 12 V.
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Note. Standard battery voltage is 12 V.
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Note. Standard battery voltage is 12 V.
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PGM-FI System
The Programmed Fuel Injection (PGM-FI) system is a sequential multiport fuel injection system.
Alternator Control
The alternator signals the PCM during charging. The PCM then controls the voltage generated by the alternator according to the electrical load determined by the electrical load detector (ELD) and driving mode. This reduces engine load to improve fuel economy.
A/C Compressor Clutch Relay
When the PCM receives a demand for cooling from the A/C system, it delays compressor engagement, and enriches the mixture to assure smooth transition to the A/C mode.
A/C Switch
The A/C (air conditioning) switch signals the PCM whenever there is a demand for cooling.
Barometric Pressure (BARO) Sensor
The BARO sensor is inside the PCM. It converts atmospheric pressure into a voltage signal that modifies the basic duration of the fuel injection discharge.
Camshaft Position (CMP) Sensor
The CMP sensor A/B input is used by the PCM to determine ignition timing at start up (cranking) and when crank angle is abnormal.
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Crankshaft Position (CKP) Sensor
The CKP sensor input is used by the PCM to determine timing for fuel injection of each cylinder and also detects engine speed and misfire.
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Engine Coolant Temperature (ECT) Sensor
The ECT sensor is a temperature dependent resistor (thermistor). The resistance of the thermistor decreases as the engine coolant temperature increases.
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Ignition Timing Control
The PCM contains the memory for basic ignition timing at various engine speeds and manifold absolute pressure. It also adjusts the timing according to engine coolant temperature.
Injector Timing & Duration
The PCM contains the memory for basic discharge duration at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final discharge duration.
By monitoring Long Term Fuel Trim, the PCM detects long term malfunctions in the fuel system, and will set a diagnostic trouble code (DTC).
Intake Air Temperature (IAT) Sensor
The IAT sensor is a temperature dependent resistor (thermistor). The resistance of the thermistor decreases as the intake air temperature increases.
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Knock Sensor
The knock control system adjusts the ignition timing to minimize knock
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Malfunction Indicator Lamp (MIL) Indication (In relation To Readiness Codes)
The vehicle has certain "readiness codes" that are part of the on-board diagnostics for the emissions systems. If the vehicle's battery has been disconnected or gone dead, if the DTCs have been cleared, or if the PCM has been reset, these codes are reset. In some states, part of the emissions testing is to make sure these codes are set to complete. If all of them are not set to complete, the vehicle may fail the test and the test cannot be finished.
To check if the readiness codes are set, turn the ignition switch ON (II), but do not start the engine. The MIL will stay on for 15-20 seconds. If it then goes off, the readiness codes are set. If it blinks several times, one or more readiness codes are not set to complete. To set each code, drive the vehicle or run the engine as described in the procedures to set them (see How To Set Readiness Codes ).
Manifold Absolute Pressure (MAP) Sensor
The MAP sensor converts manifold absolute pressure into electrical signals to the PCM.
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Primary Heated Oxygen Sensor (Primary HO2S)
The primary HO2S detects the oxygen content in the exhaust gas and sends signals to the PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The primary HO2S is installed in the exhaust manifold. By controlling the air fuel ratio with primary HO2S and secondary HO2S, the deterioration of the primary HO2S can be evaluated by its feedback period. When the feedback period exceeds a certain value during stable driving conditions, the sensor is considered deteriorated and the PCM sets a DTC.
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Secondary Heated Oxygen Sensor (Secondary HO2S)
The secondary HO2S detects the oxygen content in the exhaust gas. Downstream of the three way catalytic converter (TWC) and sends signals to the PCM which varies the duration of fuel injection accordingly. To stabilize its output, the sensor has an internal heater. The secondary HO2S is installed in the TWC.
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Starting Control
When the engine is started, the PCM provides a rich mixture by increasing injector duration.
Throttle Position (TP) Sensor
The TP sensor is a potentiometer connected to the throttle valve shaft. As the throttle position changes, the sensor varies the signal voltage to the PCM. The TP sensor is not available separately from the throttle body.
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Idle Control System
When the engine is cold, the A/C compressor is on, the transmission is in gear, the brake pedal is pressed, the power steering load is high, or the alternator is charging, the PCM controls current to the idle air control (IAC) valve to maintain the correct idle speed. Refer to the System Diagram to see a functional layout of the system.
Brake Pedal Position Switch
The brake pedal position switch signals the PCM when the brake pedal is pressed.
Idle Air Control (IAC) Valve
To maintain the proper idle speed, the IAC valve changes the amount of air bypassing the throttle body in response to an electrical signal from the PCM.
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Power Steering Pressure (PSP) Switch
The PSP switch signals the PCM when the power steering load is high.
Starter (Ignition) Switch
The starter switch signals the PCM when the engine is cranking.
Fuel Cut-Off Control
During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at engine speeds over 1,050 RPM. Fuel cut-off action also occurs when engine speed exceeds 6,400 RPM, regardless of the position of the throttle valve, to protect the engine from over-revving. When the vehicle is stopped, the PCM cuts the fuel at engine speeds over 5,000 RPM.
Fuel Pump Control
When the ignition is turned on, the PCM grounds the PGM-FI main relay which feeds current to the fuel pump for 2 seconds to pressurize the fuel system. With the engine running, the PCM grounds the PGM-FI main relay and feeds current to the fuel pump. When the engine is not running and the ignition is on, the PCM cuts ground to the PGM-FI main relay which cuts current to the fuel pump.
PGM-FI Main Relay
The PGM-FI main relay contains two separate relays. One is energized whenever the ignition is on to supply battery voltage to the PCM, power to the injectors, and power for the second relay. The second relay is energized to supply power to the fuel pump for two seconds when the ignition is switched ON (II), and when the engine is running.
The Fuel Gauge Sending Unit
The fuel gauge sending unit measures the fuel level in the fuel tank. The fuel gauge sending unit output changes according to the float position.
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Intake Air System
Refer to the System Diagram to see a functional layout of the system.
Intake Air Bypass Control System
When the engine is running, the intake air bypass control thermal valve sends air to the injectors.
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Throttle Body
The throttle body is a single-barrel side draft type. The lower portion of the throttle valve is heated by engine coolant from the cylinder head. The idle adjusting screw, which increases/decreases bypass air, is located on top of the throttle body.
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Three Way Catalytic Converter (TWC)
The TWC converts hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx) in the exhaust gas to carbon dioxide (CO2), nitrogen (N2), and water vapor.
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Exhaust Gas Recirculation (EGR) System
Refer to the System Diagram to see a functional layout of the system.
EGR Valve
The EGR valve is designed to lower peak combustion temperatures and reduce oxides of nitrogen emissions (NOx) by recirculating exhaust gas through the intake manifold and into the combustion chambers.
Positive Crankcase Ventilation (PCV) System
The PCV valve prevents blow-by gasses from escaping into the atmosphere by venting them into the intake manifold.
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Evaporative Emission (EVAP) Control System
Refer to the System Diagram to see a functional layout of the system.
EVAP Canister
The EVAP canister temporarily stores fuel vapor from the fuel tank until it can be purged into the engine and burned.
EVAP Canister Purge Valve
When the engine coolant temperature is below 147°F (64°C), the PCM turns off the EVAP canister purge valve which cuts vacuum to the EVAP Canister.
Fuel Tank Pressure (FTP) Sensor
The FTP sensor converts fuel tank absolute pressure into an electrical input to the PCM during the EVAP leak check.
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EVAP Two Way Valve & EVAP Bypass Solenoid Valve
The EVAP two way valve is installed between the fuel tank and the EVAP canister line.
The EVAP two way valve sends fuel vapor to the EVAP canister corresponding to the pressure inside the fuel tank and prevents excessive vacuum in the fuel tank by drawing in fresh air through the EVAP canister.
The EVAP bypass solenoid valve opens to bypass the two way valve when doing the EVAP leak check.
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Idle Control System Diagram
The idle speed of the engine is controlled by the idle air control (IAC) valve
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- After the engine starts, the IAC valve opens for a certain amount of time. The amount of air is increased to raise the idle speed.
- When the engine coolant temperature is low, the IAC valve is opened to obtain the proper fast idle speed. The amount of bypassed air is controlled in relation to engine coolant temperature.
Intake Air System Diagram
This system supplies air for engine needs. A resonator in the air intake duct provides additional silencing as air is drawn into the system.
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Exhaust Gas Recirculation (EGR) System Diagram
The EGR system reduces oxides of nitrogen (NOx) emissions by recirculating exhaust gas through the EGR valve and the intake manifold into the combustion chambers. The PCM memory includes the ideal EGR valve position for varying operating conditions.
The EGR valve position sensor detects the amount of EGR valve lift and sends it to the PCM. The PCM then compares it with the ideal lift in its memory (based on signals sent from other sensors). If there is any difference between the two, the PCM cuts current to the EGR valve.
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Evaporative Emission (EVAP) Control Diagram
The EVAP controls minimize the amount of fuel vapor escaping to the atmosphere. Vapor from the fuel tank is temporarily stored in the EVAP canister until it can be purged from the canister into the engine and burned.
- The EVAP canister is purged by drawing fresh air through it and into a port on the throttle body. The purging vacuum is controlled by the EVAP canister purge valve, which is open whenever engine coolant temperature is above 147°F (64°C).
- When vapor pressure in the fuel tank is higher than the set valve of the EVAP two way valve, the valve opens and regulates the flow of fuel vapor to the EVAP canister.
- During refueling, the fuel tank vapor control valve opens with the pressure in the fuel tank, and feeds the vapor to the EVAP canister.
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The vehicle has certain "readiness codes" that are part of the on-board diagnostics for the emissions systems. If the vehicle's battery has been disconnected or gone dead, if the DTCs have been cleared, or if the PCM has been reset, these codes are reset. In some states, part of the emissions testing is to make sure these codes are set to complete. If all of them are not set to complete, the vehicle may fail the emission test, or the test cannot be finished.
To check if the readiness codes are complete, turn the ignition switch ON (II), but do not start the engine. The MIL will come on for 15-20 seconds. If it then goes off, the readiness codes are complete. If it blinks several times, one or more readiness codes are not complete. To set readiness codes from incomplete to complete, do the procedure for the appropriate code.
Catalytic Converter Monitor & Readiness Code
Note. Do not turn the ignition switch off during the procedure. All readiness codes are cleared when the battery is disconnected or when the PCM is cleared with the HDS or the scan tool. Low ambient temperatures or excessive stop-and-go traffic may increase the drive time needed to switch the readiness code from incomplete to complete. The readiness code will not switch to complete until all the enable criteria are met. If a fault in the secondary HO2S system caused the MIL to come on, the readiness code cannot be set to complete until you correct the fault.
Enable Criteria
- ECT at 158°F (70°C) or higher.
- Intake air temperature (IAT) at 20°F (-7°C) or higher.
- Vehicle speed is steady, and vehicle speed sensor (VSS) reads more than 25 mph (40 km/h).
Procedure
- Connect the HDS or the scan tool to the vehicle's data link connector (DLC), and bring up the READINESS CODES for the catalyst in the DTCs menu.
- Start the engine.
- Test-drive the vehicle under stop-and-go conditions with short periods of steady cruise. After about 5 miles (8 km), the readiness code should switch from incomplete to complete.
- If the readiness code is still set to incomplete, check for a temporary DTC. If there is no DTC, one or more of the enable criteria were probably not met; repeat the procedure.
Evaporative Emissions (EVAP) Control System Monitor & Readiness Code
Note. All readiness codes are cleared when the battery is disconnected or when the PCM is cleared with the HDS or the scan tool. The enable criteria must be repeated if the intake air temperature (IAT) drops lower than 36°F (2°C) from its valve at engine start up.
- At engine start up, ECT and IAT are higher than 32°F (0°C), but lower than 95°F (35°C).
- At engine start up, the ECT and IAT are within 45°F (7°C) of each other.
- Connect the HDS or the scan tool to the vehicle's data link connector (DLC).
- Start the engine.
- Test-drive the vehicle under stop-and-go conditions with short periods of steady cruise. After about 2.5 miles (4.0 km), the readiness code should switch from incomplete to complete.
- If the readiness code is still set to incomplete, check for a temporary DTC. If there is no DTC, one or more of the enable criteria were probably not met; repeat the procedure.
HO2S Monitor & Readiness Code
Note. Do not turn the ignition switch off during the procedure. All readiness codes are cleared when the battery is disconnected or when the PCM is cleared with the HDS or the scan tool.
ECT at 140°F (60°C) or higher.
- Connect the HDS or the scan tool to the vehicle's data link connector (DLC).
- Start the engine.
- Test-drive the vehicle under stop-and-go conditions with short periods of steady cruise. During the drive, decelerate (with the throttle fully closed) for 5 seconds. After about 3.5 miles (5.6 km), the readiness code should switch from incomplete to complete.
- If the readiness code is still set to incomplete, check for a temporary DTC. If there is not DTC, the enable criteria was probably not met; repeat the procedure.
HO2S Heater Monitor Readiness Code
Note. All readiness codes are cleared when the battery is disconnected or when the PCM is cleared with the HDS or the scan tool.
- Connect the HDS or scan tool to the vehicle's data link connector (DLC).
- Start the engine, and let it idle for 1 minute. The readiness code should switch from incomplete to complete.
- If the readiness code is still set to incomplete, check for a temporary DTC. If there is no DTC, repeat the procedure.
Misfire Monitor & Readiness Code
- This readiness code is always set to available because misfiring is continuously monitored.
- Monitoring pauses, and the misfire counter resets, if the vehicle is driven over a rough road.
- Monitoring also pauses, and the misfire counter holds at its current value, if the throttle position changes more than a predetermined value, or if driving conditions fall outside the range of any related enable criteria.
Fuel System Monitor & Readiness Code
- This readiness code is always set to available because the fuel system is continuously monitored during closed loop operation.
- Monitoring pauses when the catalytic converter, EVAP control system, and HO2S monitors are active.
- Monitoring also pauses when any related enable criteria is not being met. Monitoring resumes when the enable criteria is again being met.
Comprehensive Component Monitor & Readiness Code
This readiness code is always set to available because the comprehensive component monitor is continuously running whenever the engine is cranking or running.
EGR Monitor & Readiness Code
Note. Do not turn the ignition switch off during the procedure. All readiness codes are cleared when the battery is disconnected or when the PCM is cleared with the HDS or the scan tool.
ECT at 176°F (80°C) or higher.
- Connect the HDS or the scan tool to the vehicle's data link connector (DLC).
- Start the engine.
- Drive at a steady speed with the transmission in D position, 50-62 mph (80-100 km/h) or above for more than 10 seconds.
- With the transmission in D position, decelerate from 62 mph (100 km/h) or above by completely releasing the throttle for at least 5 seconds. If the engine is stopped during this procedure, go to step 3 and do the procedure again.
- If the readiness code is still set to incomplete, check for a temporary DTC. If there is no DTC, the enable criteria was probably not met; repeat the procedure.