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An engine diagnostic is a test which is performed to learn more about why an engine is not functioning properly. The engine diagnostic is used to gather data which can be utilized in the repair of the vehicle, and it can also be used during things like emissions testing, in which a car is checked to confirm that it adheres to emissions standards for vehicles in its class.

Performing an engine diagnostic starts with plugging a device into the car’s onboard computer. There is usually a space to do this just under the dashboard on the driver’s side of the car. The device interfaces with the computer, returning a diagnostic code which can be referenced against a list of codes for vehicles from that manufacturer. If the engine is in good working condition, the device will inform the user. If there is a problem, one or several codes may be displayed.

Codes can mean a wide variety of things. During an engine diagnostic, the mechanic determines what is causing each code to display, for the purpose of developing an estimate to let a driver know how much it will cost to address a problem. Sometimes, the issue is simple, while in other cases, it can be very complex.

People usually bring cars in for an engine diagnostic when the “check engine” light is displaying, or when they are experiencing car problems. It can be helpful for the mechanic to know which kinds of problems are being experienced, and how long the driver has noticed the problems. For things like the check engine light, sometimes the light goes off when nothing is really wrong; for example, some cars will display the light when it is time for an oil change to goad drivers into going to the mechanic, and the mechanic can clear the alarm so that it will stop displaying.

Devices to perform engine diagnostics are available through auto repair stores. Having the device is not terribly useful without having access to a database of codes. Some automotive repair websites maintain code databases which people can use for reference while working on their cars. People can use this information to repair their cars, or simply to determine whether or not a car needs to be brought in to a mechanic.

People should be aware that sometimes clearing codes and alarms can cause a car to fail emissions testing. For example, if someone clears the check engine light without addressing the problem which caused it to illuminate, the car will fail emissions testing.


We offer a full range of workshop services to vehicle owners in Nowra and the South Coast region. We can help you with everything from an oil change to an engine change. We can handle any problem on both foreign and domestic vehicles.

  • We make auto repair more convenient for you
  • We are a friendly and professional group of people
  • We handle a wide range of car services
  • Same day service for most repairs and maintenance
  • We get the job done right — the first time


  • I have a warning light on my dash, what happens if I ignore it?

    Your vehicle may enter ‘limp mode’ with limited drivability. Continuing to drive with the problem unrepaired could damage components of the vehicle, or could result in poor fuel consumption. It is advisable to visit us for a check as soon as you notice the warning light.

  • Can I still use the service, even if I don't have a warning light?

    YES! The tool can even reset the service interval warning lights if you’ve completed a DIY service. Some modern vehicles need a diagnostic tool to activate the wiper blade change position, as well as needing the tool to inform the vehicles’ ECU that a new battery has been installed. It can also help diagnose any problems which may be causing the car to run poorly, or suffer from poor fuel consumption.

  • What is the most common check engine light problem?

    Although a check engine light can come back with any number of engine faults, the most common problem tends to be the Oxygen Sensor. This sensor can have a drastic effect on your cars ability to run smoothly, and can affect your fuel economy potentially by up to 40%. Learn more about Oxygen Sensors below.

  • What is an Oxygen Sensor?

    Oxygen sensors are a critical component of your engines operating system. Fitted to most engines since 1980 and every new car off the factory floor today, they can be overlooked at general service intervals with detrimental results. The oxygen sensor is a physical sensor mounted inside the exhaust where it is able to obtain a reading as part of the emission control system. This information is then relayed back to the engine management computer where it can determine if it’s running as efficiently as possible while taking emissions control into consideration. Every engine has a perfect ratio of fuel and air delivery which allows it to run as well as possible. If your engine isn’t running within this ratio it may be running either too rich or too lean.

    Rich – Rich is when there is less air being delivered then your engines perfect ratio. This means that a rich engine will have more fuel left over after combustion. This unused fuel also creates a greater level of pollution

    Lean – Contrary to rich, if there is more air then required being allowed into the engine, this excess oxygen will throw out the perfect ratio and your car will suffer. If your motor is running lean, not only will you experience poor performance, you may cause damage to your engine.

    How do I know if my Oxygen sensor is faulty?

    Here are some of the more common effects a faulty unit may have on your car:

    Sudden drop in fuel economy – If the oxygen sensor fails, the engine is unable to obtain a clear understanding of how much fuel it needs and as a result, additional fuel can be pumped through into your motor.

    Drastic change in engine performance – The oxygen sensor plays such a critical part in your cars fuel delivery system, a faulty sensor will be unable to correctly determine the volume of fuel required. This will result directly in lack of engine performance that will be quite noticeable on your daily drive.

    Harsh idling – Idling will also be affected due to changes in fuel delivery. The effects can alter from significantly rougher idling, to sporadic fluctuations in idling behaviour. This should be quite noticeable and therefore one of the easier methods of detection.

    Strong smell of fuel – If the engine is running rich, there will be more fuel being pumped into the engine and a distinctive fuel smell may be present from either the exterior or interior of your car.


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