System Details

Click on the subsystems below to learn more about the subsystems monitored through OnStar Vehicle Diagnostics.* Please note the capabilities of OnStar Vehicle Diagnostics and On-Demand Diagnostics checks vary by make and model.

Engine and/or Transmission System	Emissions Control System
Air Bag System	Antilock Brake System
StabiliTrak® System	OnStar System

Engine and/or Transmission System

This system is responsible for making the power necessary to move the vehicle and sending that power to the wheels.

NOTE: If your vehicle's Check Engine/Service Engine Soon light is on or not working properly your vehicle requires service and may not pass your state's emission inspection program (if one exists).

[+] Air Induction System

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Air Induction System
Your engine uses fuel and air to create the power to drive the vehicle. To make your vehicle run at peak efficiency, several sensors send information to the engine computer about the air that is flowing into the engine. When you start your vehicle, the diagnostic system runs a check on the sensors, and continues to monitor these sensors to ensure your engine is operating normally. In the event that one of the sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Fuel Management System

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Fuel Management System
The engine computer uses the air induction system measurements and other sensors to determine how much fuel is needed to make the engine run at peak efficiency. The fuel management system controls and monitors things like the fuel injectors, fuel pressure, and fuel level. In the event that one of the sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Engine Cooling System

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Engine Cooling System
In order for the engine to continually run at peak efficiency, no matter what the outside temperature is, the engine temperature must constantly be monitored and controlled. The engine computer uses several sensors throughout the cooling system to monitor temperatures, and can make adjustments to engine fan speed to keep the engine at the optimum temperature. In the event that one of the sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light. That information can also be sent to your OnStar Vehicle Diagnostic check to alert you that service is needed.

[+] Throttle Control System

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Throttle Control System
When you push on the accelerator pedal, a signal is sent to the engine to raise the engine speed. This can be accomplished through a throttle cable or, on some vehicles, by using sensors and small electric motors. Your engine computer uses several sensors to constantly monitor every movement of the accelerator pedal and the position of the throttle. This system can also work in conjunction with your Antilock Brake system and StabiliTrak® features to reduce engine power in certain driving situations. In the event that one of the sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon or Reduced Power light or message. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Variable Valve Timing System (if equipped)

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Variable Valve Timing System (if equipped)
Air and fuel enter the engine, and exhaust gases exit through the intake and exhaust valves. The engine camshaft controls when the valves open and how long they stay open. Engines with variable valve timing use sensors to allow the engine computer to change the position of the camshaft. This changes the opening and closing of the valves so that the engine can give higher performance and better efficiency. In the event that one of the sensors within this system detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Displacement on Demand™/Active Fuel Management System™ (if equipped)

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Displacement on Demand™/Active Fuel Management System™ (if equipped)
Some vehicles may be equipped with this fuel-saving feature. The system uses the engine computer to monitor the vehicle's power demand. When less power is required, such as when cruising at a steady speed, the system will operate using only half of the engine cylinders. This allows your vehicle to achieve better fuel economy. When more power is needed, such as when accelerating, the system will maintain full cylinder operation. In the event that the engine computer senses a problem with this system, the Check Engine/Service Engine Soon light may illuminate. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Ignition System

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Ignition System
As the proper amount of fuel and air is sent into the engine, the mixture is ignited inside the engine cylinders to create power. In gasoline engines, this is accomplished using spark plugs and an ignition system. Several engine sensors tell the computer when to create the spark so that peak engine efficiency and performance is obtained. In the event that one of the sensors in the system detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Engine Electrical System, including the Charging and Starting System

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Engine Electrical System, including the Charging and Starting System
The engine electrical system has several parts. The starting system uses an electric motor attached to the engine and is powered by the battery. In order to ensure that your vehicle starts, the vehicle battery needs to be continually charged. This is done using a device called a generator (or alternator). Once the engine is running, the generator creates electrical energy to recharge the battery. The generator and battery work together to supply the power to run the electronic features in your vehicle. In the event that one of the sensors in the system detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon or Battery light to indicate a malfunction. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Hybrid Powertrain System (if equipped)

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Hybrid Powertrain System (if equipped)
If your vehicle has a hybrid powertrain option, it is designed to be more fuel efficient than a conventional powertrain. Several additional electrical and mechanical devices work in conjunction with the engine, transmission, and electrical system to achieve this. The main engine computer is constantly monitoring several other hybrid system computers and sensors to ensure proper operation. In the event that one of the sensors in the system detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light or other display messages to indicate a malfunction. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Transmission System

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Transmission System
Power created by the engine is transferred to the vehicle drive wheels through the transmission. An automatic transmission uses fluid pressure, gears, and sensors to control power and automatically shift gears. Several sensors keep track of transmission functions. In the event that one of the sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light to indicate a malfunction. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Four-Wheel Drive System (if equipped)

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Four-Wheel Drive System (if equipped)
If your vehicle has four-wheel drive, you can send the engine power to all four wheels for extra traction. This is accomplished with a device called a transfer case. In the event that one of the system sensors detects a problem, the transfer case computer can turn on the Service 4WD light to indicate a malfunction. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

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Emissions Control System

This system is responsible for limiting the emissions from the vehicle, both by keeping fuel and its vapor contained in the fuel system and by limiting exhaust tailpipe emissions by ensuring efficient engine operation.

NOTE: If your vehicle's Check Engine/Service Engine Soon light is on or not working properly your vehicle requires service and may not pass your state's emission inspection program (if one exists).

[+] Evaporative Emissions System

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Evaporative Emissions System
The purpose of the evaporative emissions system is to contain fuel vapors from the fuel tank and fuel system. When you fill up your fuel tank and tighten the fuel cap, the fuel system is sealed and the system ensures that any fuel vapors are used by the engine and kept out of the atmosphere. In the event that this system has a problem, sensors within the system can tell the engine computer to turn on the Check Engine/Service Engine Soon light, Check Gas Cap light, or another display message to indicate a malfunction. If the fuel cap is not properly tightened, the system can detect this and can also cause the Check Engine/Service Engine Soon light, Check Gas Cap light, or another display message to come on. It is important to ensure that you tighten the fuel cap after refueling. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service may be needed.

[+] Exhaust Emissions System

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Exhaust Emissions System
The engine exhaust emissions system uses sensors to monitor engine combustion to ensure the engine always runs at peak efficiency, producing the least amount of pollutants. By closely monitoring the engine exhaust and providing feedback to the engine computer, the engine can adjust performance to ensure it is producing the least amount of emissions possible. In the event that one of the system sensors detects a problem, the engine computer can turn on the Check Engine/Service Engine Soon light to indicate a malfunction. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

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Air Bag System

This system is responsible for monitoring and deploying the air bag system.

[+] Air Bag Deployment System

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Air Bag Deployment System
In an impact of sufficient severity, the air bag(s) can deploy. This system continually checks and monitors the deployment devices to ensure they are operating properly. In the event that a malfunction is detected, the computer can turn on the Air Bag Readiness light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed. Your GM owner's manual contains more information about this system.

[+] Collision Sensor System

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Collision Sensor System
Your vehicle has electronic sensors that can distinguish between a moderate crash impact and a severe crash impact. In the event of a crash, these sensors send information to the computer that controls air bag deployment. In the event that a malfunction is detected, the computer can turn on the Air Bag Readiness light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Safety Belt Pretensioner System (if equipped)

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Safety Belt Pretensioner System (if equipped)
Your vehicle may have safety belt pretensioners for the driver and right front passenger safety belts. This system is designed to work with the air bag system. They help the safety belts reduce an occupant's forward movement in a moderate to severe frontal, near frontal, rear, or side crash, or in a rollover. In the event that a malfunction is detected, the computer can turn on the Air Bag Readiness light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

[+] Passenger Presence System (if equipped)

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Passenger Presence System (if equipped)
The passenger sensing system works with sensors that are part of the right front passenger's seat and safety belt. Children and small adults can be injured by the force of an air bag deployment. The sensors are designed to detect the presence of a properly seated occupant and determine if the passenger's air bag should be deployed. In the event that a malfunction is detected, the computer can turn on the Air Bag Readiness light and/or Passenger Air Bag light. That information can also be sent to your OnStar Vehicle Diagnostics check to alert you that service is needed.

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StabiliTrak^® System

The StabiliTrak^® System is an advanced computer controlled system that can help the driver maintain directional control of the vehicle in difficult driving conditions. This is accomplished by using input from vehicle motion sensors and the steering wheel position to selectively apply any one of the vehicle's brakes and, if necessary reduce engine power.

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Actual image may vary.

When the system activates, a Stability Active light or message may turn on. This is normal operation. If the system detects a problem, the Stability Disabled or Service Stability light or message will be displayed. If the light or message remains on, even after pressing the Traction/Stability selector button, your vehicle may require service. That information will also be sent in your OnStar Vehicle Diagnostics email to alert you that service may be needed.

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Important: OnStar Vehicle Diagnostics capability is limited to certain key operating systems and cannot monitor and/or detect all operating components. The systems that OnStar does not check include, but are not limited to, things such as your engine oil level and other fluid levels, including transmission and fuel, as well as wear-sensitive maintenance items, including tires, belts and brake pads, filters, and other mechanical operating components such as your fuel pump, starter, battery, etc. This service does not replace the need to maintain your vehicle in accordance with your vehicle manufacturer's recommended maintenance schedule. For more information, visit call 1.888.4.ONSTAR or press your blue OnStar button.

*Available on 2004 MY & newer GM models equipped with the GM Oil Life System. Diagnostics not available on Pontiac Vibe, Saab 9-3 and 9-5. Diagnostic services vary on Cadillac SRX V8, Saturn VUE, Ion and L-Series, Chevy Silverado diesel and GMC Sierra diesel. For details, call 1.888.4.ONSTAR (1.888.466.7827) or press your blue button.