• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
• Review Strategy Based Diagnosis for an overview of the diagnostic approach.
• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

Symptoms cover conditions that are not covered by DTCs. Certain conditions can cause multiple symptoms. These conditions are listed together under Symptoms Testing. Conditions that may only cause specific symptoms are listed separately under additional symptoms testing. Perform the symptoms testing before using the additional symptoms testing.

Before using the Symptom tables, perform the following inspections:

• Verify that the MIL is operating correctly. Use the scan tool to command the lamp ON and OFF.
• Verify that there are no DTCs that are stored.
• Verify that the scan tool data is within a normal operating range. Refer to Control Module References for scan tool information.
• Verify that the vehicle tires are correctly inflated and meet original equipment specifications.
• Verify that the Flex Fuel reservoir has a sufficient amount of fuel in it.
• Verify the customer concern.
• Perform the Visual/Physical Inspection in this section. The visual/physical inspection is extremely important, and can lead to correcting a condition without additional testing. It may also help reveal the cause of an intermittent condition.

Many intermittent conditions occur with harness or connector movement due to engine torque, rough pavement, vibration or physical movements of a component. Refer to the following for a list of issues that may cause an intermittent condition:

• Moisture and water intrusion in connectors, terminals, and components
• Incomplete connector mating
• Poor terminal contact
• High circuit or component resistance—High resistance can include any resistance, regardless of the amount, which can interrupt the operation of the component.
• Harness that is too short or tight
• Wire insulation that is chaffed or cut
• High or low ambient temperature
• High or low engine coolant temperatures
• High underhood temperatures
• Heat build up in component or circuit due to circuit resistance, poor terminal contact, or high electrical load
• High or low system voltage
• High vehicle load conditions
• Rough road surfaces
• Electromagnetic Interference (EMI)/circuit interference from relays, solenoids or other electrical surge
• Incorrect installation of aftermarket, add on accessories

• Verify that the control module grounds are clean, tight, and correctly located.
• Verify that the vacuum hoses are not split or kinked, and are properly connected, as shown on the vehicle emission control information label.
• Verify that the air filter is clean and free from restrictions.
• Verify that there is no water intrusion in connectors terminals and components.
• Inspect the air intake ducts for the conditions listed below:
• Collapsed
• Damaged areas
• Looseness
• Incorrect installation
• Leaking

  If any condition listed above has allowed non-metered air to enter the air induction system, the Throttle Body Idle Air Flow Compensation Reset procedure must be performed after the repair is complete.

• Inspect for air leaks at the throttle body mounting area, the Mass Air Flow (MAF) sensor and intake manifold sealing surfaces.
• Inspect the wiring harness for the following conditions:
• Poor connections
• Pinches
• Cuts
• Inspect for loose, damaged, unseated, or missing sensors/components.
• Inspect the terminals for corrosion and correct contact.

1. Test for the following conditions:
• Incorrectly operating fuel system. Test/inspect for the following:
• Incorrect system operation or fuel pressure. Refer to Fuel System Diagnosis.
• Leaking or incorrectly operating fuel injectors. Refer to Fuel Injector Diagnosis.
• Contaminated or poor quality fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
• Incorrectly operating ignition system. Test/Inspect for the following:
• Spark plugs with incorrect heat range or an abnormal condition. Refer to Ignition System Specifications.
• Coolant or oil fouled spark plugs. For diagnosis, refer to Oil Consumption Diagnosis and Coolant in Combustion Chamber.
• Secondary ignition system susceptible to moisture. Wet down the secondary ignition system with water from a spray bottle to help locate damaged or deteriorated components. Look/listen for arcing or misfiring as the water is applied.
• Weak spark using the EL-26792 HEI Spark Tester. For equivalent regional tools, refer to Special Tools. Refer to Electronic Ignition System Diagnosis.
• Non-metered air flow into the air induction system. If a leak is found, repair as necessary. The Throttle Body Idle Air Flow Compensation Reset procedure must be performed when the repair is complete.
• An incorrectly installed MAF/Intake Air Temperature (IAT) Sensor. Refer to Mass Airflow Sensor Replacement.
• An incorrectly operating transmission Torque Converter Clutch (TCC). The scan tool should indicate an engine speed drop when the TCC is commanded ON.
• An incorrectly operating A/C compressor
• Items that can cause an engine to run lean or rich. Refer to DTC P0171 or P0172.
• Split or kinked vacuum hoses. Verify that the routing and connections are as shown on the vehicle emission control information label.
• Excessive knock sensor system spark retard activity. Refer to DTC P0324-P0333, P06B6, or P06B7 and Knock Sensor Replacement.
• An incorrectly operating turbocharger system, if equipped.
• Leaking turbocharger charge air cooler system, if equipped.
• Electromagnetic interference on the reference circuit, which can cause a misfire condition. You can usually detect electromagnetic interference with a scan tool by monitoring the engine speed parameter. A sudden increase in the engine speed parameter with little change in actual engine speed indicates that electromagnetic interference is present. Inspect the high voltage components near the ignition control circuit if a condition exists.
• An incorrectly operating positive crankcase ventilation system. Inspect all connections for leaks or restrictions. Refer to Crankcase Ventilation System Inspection/Diagnosis
• A stuck open Evaporative Emission (EVAP) Canister Purge Solenoid Valve.
• An incorrectly operating variable camshaft actuator system. Engine warm and idling, command the intake and exhaust camshaft actuators from 0 degrees to 20 degrees and back to zero while observing the scan tool Intake Camshaft Position Variance and Exhaust Camshaft Position Variance parameters. Each parameter should be less than 2 degrees in each of the commanded states.
• If any of the parameters is greater than 2 degrees, inspect the suspect camshaft actuator and camshaft actuator solenoid valve and valve bore for contamination, obstruction, and damage. Refer to Camshaft Actuator System Description , Camshaft Position Actuator Solenoid Valve Replacement - Exhaust and Camshaft Position Actuator Solenoid Valve Replacement - Intake.
• An incorrectly operating engine cooling system. Test/Inspect for the following:
• A thermostat with incorrect heat range. Refer to Thermostat Diagnosis.
• Incorrectly operating engine coolant thermostat heater, if equipped. Refer to DTC P0597–P0599.

  Incorrect engine coolant level. Refer to Cooling System Draining and Filling.

• An incorrectly operating exhaust system. Test/Inspect the components for the following:
• Physical damage or possible internal malfunction.
• Restricted 3-way catalytic converters.

  For more information, refer to Symptoms - Engine Exhaust.

• An engine mechanical condition. Test/Inspect for the following:
• Too much oil in the combustion chamber or leaking valve seals
• Incorrect cylinder compression
• Sticking or leaking valves
• Worn camshaft lobes
• Incorrect valve timing
• Broken valve springs
• Too much carbon buildup in the combustion chambers. Clean the chambers with Top Engine Cleaner, if necessary. Follow the instructions on the can.
• Incorrect engine parts

  For additional information, refer to Symptoms - Engine Mechanical.

2. If the above conditions do not address the symptom, refer to the additional symptoms tests.

• Test the engine for an overheating condition. Refer to Symptoms - Engine Cooling.
• Inspect for excessive carbon buildup in the combustion chambers. Clean the chambers with Top Engine Cleaner, if necessary. Follow the instructions on the can.
• If there are no engine mechanical faults, fill the fuel tank with a known high quality fuel that meets the vehicle minimum octane requirements.

Under certain conditions the engine control module may limit engine power by reducing engine torque and, for some vehicles, fuel pressure as well. For most, but not all of the conditions, the engine control module will illuminate the reduced engine power lamp on the instrument cluster. If equipped with the driver information center feature, a reduced engine power message may be displayed as well. A DTC may not be set.

A repair may not be necessary. Observe the scan tool Reduced Engine Power History parameter or refer to K20 Engine Control Module: Scan Tool Information to determine the reason for the reduced engine power event.

Verify or inspect for the following:

• Vehicle being operated at sustained high engine speeds, or, towing heavy loads up an incline for an extended period of time, which may cause the engine oil or coolant to overheat. Inspect the airflow passageways in front of the engine for obstructions and clear away any debris or foreign material that is found. If no obstructions are found, review approved driving habits with the customer. The customer may need to operate the vehicle at a higher engine speed to improve cooling system performance, or, at a slower engine speed to reduce engine load.
• A cooling fan condition which may cause the ECM to reduce engine power. Refer to Cooling Fan Description and Operation and Cooling System Description and Operation to verify correct operation of the cooling fan.
• Fuel temperature out of range too high.
• High pressure fuel system condition. Refer to Fuel System Diagnosis and Fuel System Description to verify high pressure fuel system operation.
• Reduced engine power due to OnStar® remote command. Verify the vehicle is not in the OnStar® initiated Stolen Vehicle Slowdown mode.
• Turbocharger boost pressure out of range, if equipped. Refer to DTC P0234 or P0299.
• Intake air temperature out of range too high. Engine running, observe the scan tool IAT sensor parameter. The readings should be between −40° C and +149° C (−40° F and +300° F. Refer to DTC P0112, P0113, or P0114 for additional information.
• If equipped with manual transmission, excessively hot manual transmission fluid. Under severe operating conditions the fluid may become overheated, which may result in ECM commanded reduced engine power. Allow the transmission and fluid to cool down. Review the maintenance recommendation in the owners manual to determine if the transmission fluid is due to be serviced.
• If equipped with automatic transmission, an incorrect learned brake pedal position (BPP) sensor. Verify the following actions have not occurred:
• An engine control module has been replaced
• A BPP sensor that has been replaced
• A BPP sensor is misaligned or incorrectly installed

  If any of the above actions have occurred, the Brake Pedal Position Sensor Calibration procedure must be performed.

• Inspect for leaking, damaged, or deteriorated fuel lines and components.
• Inspect for a saturated EVAP canister. Refer to Evaporative Emission Control System Description.
• Inspect for a condition with the internal components of the fuel tank assembly. Refer to Fuel System Description.

• Observe the scan tool Throttle Body Idle Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning.
• Test the engine coolant temperature (ECT) sensor. Compare the ECT sensor value to the intake air temperature (IAT) sensor value on a cold engine. The ECT and IAT sensor values should be within approximately 3°C (5°F). If the ECT sensor is out of range with the IAT sensor, test the resistance of the ECT sensor. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor and Temperature Versus Resistance - Intake Air Temperature Sensor for resistance specifications. Replace the ECT sensor if the resistance is not within specification. If the sensor is within the specification, test the ECT circuits for a high resistance.
• Verify that the fuel system operates correctly and provides adequate pressure for engine start-up The fuel pump should turn ON for 2 s when the ignition is turned ON. Refer to Fuel System Diagnosis and Fuel System Description for fuel pressure specifications and additional information.

• Test the fuel pressure. Refer to Fuel System Diagnosis.
• Test the manifold absolute pressure (MAP) sensor. Refer to DTC P0106, P0107, or P0108.
• Test the generator. Refer to Symptoms - Engine Electrical. Repair the charging system if the generator output voltage is less than 9 V or more than 16 V.

• Inspect the engine electrical system for incorrect operation. Refer to Symptoms - Engine Electrical.
• Verify that each injector harness is connected to the correct injector.
• Inspect the MAF sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor Replacement.
• If equipped with a Supplemental Transmission Auto Shift , verify that the brake sensor is not stuck in the applied position. The Enhanced Electronic Pedal Override functionality in the ECM reduces the driver torque request if the accelerator pedal and the brake pedal are applied simultaneously
• Verify the correct operation of the turbocharger system. Perform the following:
• Inspect the turbocharger moving components for looseness, binding, wear and damage.
• Verify that the turbocharger air inlet ducts and outlet ducts are tightly sealed and that the air passages are not obstructed. Engine running, use a smoke generating device or a solution of dish soap and water in a spray bottle to pinpoint any air leaks.
• Inspect the turbocharger system for external fluid leaks. Inspect the gaskets and sealing surfaces and look for cracks in the turbocharger components.
• Inspect for excessive turbocharger system noise. Operate the vehicle at various acceleration modes and compare the turbocharger noise to the noise made by a known good system.
• For additional information, refer to Compressor Air Intake Turbocharger Cleaning and Inspection.

• Inspect the engine electrical system for incorrect operation. Refer to Symptoms - Engine Electrical.
• Verify that each injector harness is connected to the correct injector.
• Inspect the MAF sensor for obstruction, contamination, and damage. Refer to Mass Airflow Sensor Replacement.

• Observe the scan tool Throttle Body Idle Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. Inspect the throttle body and bore and clean, if necessary. Refer to Throttle Body Inspection and Cleaning.
• Inspect for heavy loads being carried or towed
• Inspect for acceleration rate too much or too often
• Inspect for incorrect operation of the speedometer.
• Inspect the brake system for brake drag.

• An excessively high idle may be due to the floor mat interfering with the accelerator pedal. With this condition present, it may not be possible to shift the transmission into gear. Inspect the accelerator pedal for binding and verify that the floor mat is not interfering with the accelerator pedal movement.
• Engine idle speed be unstable or the engine may stall if the ECM has learned an incorrect idle/airflow compensation value. A DTC may also set. Observe the scan tool Throttle Body Idle Airflow Compensation parameter. A value greater than 90% may indicate an excessive accumulation of deposits in the throttle bore. If the throttle body needs cleaning, refer to Throttle Body Inspection and Cleaning.

  The actions listed below may also cause the ECM to learn an incorrect idle value:

• The ECM has been replaced
• The throttle body has been replaced
• The throttle body has been cleaned but the idle learn procedure was not performed after completing the cleaning
• The air induction system was leaking, allowing non-metered air to enter the combustion chamber. The leak was repaired, but the idle learn procedure was not performed after repairing the leak.

  If any of the actions listed above have occurred, the Throttle Body Idle Air Flow Compensation Reset procedure must be performed.

• Inspect the engine mounts for looseness, damage, and wear.
• Inspect the intake and exhaust manifolds for casting flash.
• Inspect engine coolant for a slushy or frozen condition. Refer to Cooling System Draining and Filling.

• Verify that each injector harness is connected to the correct injector.
• Inspect for slow responding Heated Oxygen Sensors (HO2S). The HO2S should respond quickly to a change in throttle position. If the HO2S do not respond to different throttle positions, inspect for contamination from fuel, silicon, or the incorrect use of room temperature vulcanizing sealant. The sensors may have a white powdery coating and result in a high, but false, signal voltage, which gives a rich exhaust indication. The ECM reduces the amount of fuel delivered to the engine, causing a driveability condition.