mil check engine light service engine diy repair

Learn how to diagnose the Check Engine Light yourself using an OBD II scanner that costs less than $20. 

  • What it means when check engine light turns on. 
  • Most common check engine light problems.
  • How to diagnose check engine light yourself using a $20 OBD-II code reader

No need to panic or spend hundreds of dollars if your check engine light turns on. In this article, you will learn what the Check Engine Light is telling you. 

srs-brake-check-engine-service-light Your vehicle?s check engine light is part of its onboard diagnostics (OBD) system. When the Engine Control Unit (ECU), which is the engine?s computer, finds a problem it can not adjust automatically, it will set a fault code in its memory and turn on a warning indicator on your dashboard. An illuminated check engine light can mean there is a problem with any number of components in your vehicle. It could be something as simple as a loose gas cap, or an indication that something is out of specification, or a sign that something is completely broken. After the code is read, the problem can be pinpointed, and an action plan can be put in place to repair it.

Depending on the manufacturer, the warning on the dashboard may say one of the follwoing but they all refer to the Check Engine Light. 

  • Service Engine Soon
  • Check Engine
  • Check - Engine Symbol

What Causes the Check Engine Light to come on?

The check engine light illuminates when a fault code has been set in the ECU?s memory. When this code has been stored, it turns on a warning light on your dashboard, letting you know that something is wrong and it needs to be investigated and addressed. There may be more than one error code stored, depending on what the problem is.

Info! Keep in mind: if the check engine light is blinking, it means it is a critical problem that needs to be addressed right away. If this happens, find a safe place to pull over, and call a tow truck right away to have the vehicle towed to a repair shop. If the check engine light is blinking, it is not safe to drive the car.

All modern cars manufactured in 1996 or later include a common diagnostic system called OBD II. This OBD II system provides performance diagnostics about your vehicle and provides access to check engine codes. Check engine codes can be accessed with any OBD II scanner.

Five Common Reasons Your Check Engine Light Is On

Although there are many reasons for the check engine light to be illuminated, some reasons are much more common than others. The following is a list of the most frequent causes and the possible consequences of not repairing it right away.

Oxygen Sensor

  • check engine light faulty oxygen sensorFrom time to time, one of the oxygen sensors in the exhaust system will need to be replaced. Depending on the vehicle, they are located at various points before and after the catalytic converter. Its job is to measure the amount of unburned oxygen in the exhaust system. The only way you can discover if it has failed, unfortunately, is for the error code to set in the ECU and subsequently, the check engine light will come on.

    • If the problem with the oxygen sensor is not addressed immediately, you will experience decreased fuel economy because your engine will be burning more fuel than necessary.

Catalytic Converter

  • check engine light bad cataltyic converterYour vehicle?s catalytic converter converts carbon monoxide into carbon dioxide, which is then emitted into the atmosphere through the exhaust. When the catalytic converter malfunctions, it is usually due to an underlying problem, such as neglected maintenance with an oxygen sensor, spark plugs, or other parts of the engine.

    • If the catalytic converter is not replaced in a timely manner, you will experience reduced engine performance, decreased fuel economy, and your vehicle will not pass an emissions test, necessary for state inspections.
    • Replacing the catalytic converter can cost between $250 for an universal cat up to $1,500 for OEM cataltyic converter that you guy at the dealer. 
    • Related Fault Code: P0420 most common. Can also trigger missfire codes, see spark plug section below. 

Spark Plugs/Wires

  • Spark plugs are responsible for providing the necessary spark to ignite the air/fuel mixture in your vehicle?s engine. When they malfunction, you will experience poor fuel economy and reduced performance.

    • Spark plugs that are too worn can cause clogged catalytic converters and damaged oxygen sensors.
    • The cost to replace spark plugs can vary between $100 for a 4 cylinder car up to over $350 for V8 engine that may have double spark plugs. The spark plugs themselves are very inexpensive you can check spark plugs prices here
    • Related fault codes: P0300, P0301, P0302, P0303, P0304, P0304, P0305, P0306, P0316 

Mass Airflow Sensor 

  • check engine bad mass air flow sensorThe mass air flow sensor?s job is to measure the amount of air coming into the engine so that the appropriate amount of fuel can be injected and the correct air/fuel mixture can be achieved. This results in efficient engine operation.

    • Damaged mass air flow sensors can result in damage to oxygen sensors, spark plugs, or catalytic converters. Poor fuel economy and reduced performance will result if it is not replaced.
    • The cost to replace the mass air flow sensor varies between $150-$300. You can usually find replacement mass air flow sensors on Amazon for much less than what you would pay at the dealer. 
    • Related fault codes: P0102, P0101, P0103, P0171

Gas Cap

  • check-engine-light-loose-gas-cap It sounds silly, but a loose or missing gas cap can cause the check engine light to come on. Not only is it necessary for sealing the fuel tank, but it also helps to maintain the appropriate amount of pressure in the fuel system. Without the cap, the amount of air pressure is affected, setting the error code in the ECU?s memory.

    • If you procrastinate in replacing it, you will lose more fuel through evaporation.
    • The cost to get a new gas cap is less than $30. Look up for a replacement gas cap on Amazon
    • Related fault codes: P0455, P0457, P0442

How to scan the check engine light. Hint: OBD II Scanners

If you repair your vehicle, or if you own multiple vehicles and enjoy tinkering with them, it may make sense to purchase an OBD II scanner. This will enable you to read error codes yourself and have a better understanding why the check engine light is on. OBD II Scanners, often referred as Code Reader are sold at retailers and auto repair shops and range in price from $50-$200. There are several very popular code readers that cost under $25 which you can buy from Amazon. If you are looking for a code reader we recommend you take a look at Amazon's Best Selling OBD-II Code Readers


obd-ii-port-scanner scan read check engine lightThe scanner is plugged into the OBD port in your vehicle, and it subsequently will display what the trouble code is. From there, you can research the code and discover the part of the car where the problem is occurring, then take the necessary steps to repair it.

If purchasing a scanner is not an option, but you still would like to self-diagnose the problem, many automobile repair shops or retail stores (such as AutoZone) have OBD code reader devices and will read the code stored in the vehicle?s memory for free. The OBD fault code is composed of five characters. It begins with the letter P and is followed by four numbers. Here is a breakdown of an OBD Code:

  • P - stands for Powertrain, and is standard for all codes. It will always be the first letter that appears.

  • The second character, which will be a 0, 1, 2, or 3, indicates whether the problem is generic, ?0?, or manufacturer specific, ?1, 2, 3?.

  • The third character indicates which system is affected. So this could be the emissions, the transmission, the fuel system, etc.

  • The fourth and fifth numbers will vary and indicate which particular component of the defined system is being affected.

    How to Repair a Check Engine Light

    Repairing an illuminated check engine light depends on what the problem is that is causing the light to come on. You can read the OBD II codes with a handheld scanner that you can purchase yourself, which might make sense if you own a lot of vehicles or enjoy repairing them.

    Alternatively, you can take the car to an auto parts retailer and oftentimes, they will read the codes for you at no charge. Once the error codes have been read and interpreted, then measures can be taken to repair the problem.

    Once the problem is fixed, the OBD II scanner is used to clear the codes from memory. When the car is started again, the check engine light should no longer be illuminated.

    Cost of Repairing a Check Engine Light

    Again, the full cost of repairing a check engine light depends on where the problem exists. It also depends on the type of vehicle you have and the hourly labor rates at your repair shop.

    Replacing a malfunctioning catalytic converter is one of the more expensive repair jobs, as the part alone will cost you between $600-$1000 or more. Labor costs for this type of job range between $150-$250, depending on the repair shop?s hourly labor rate.

    The total cost of replacing a spark plug, labor and parts included, is an average between $200-300 depending on the repair shop you use and the type of vehicle you have. Replacing a spark plug is a fairly easy job to do on your own if you want to save money on labor costs. Replacing an oxygen sensor is about the same cost as the spark plug replacement.

    The average cost of replacing a mass air flow sensor, parts and labor included, is about $300-$400, depending on the repair shop and vehicle type.

    Overall costs will vary depending on the specific problem. It will be more expensive if the situation is more critical (such as if the light is blinking) versus something that can be addressed later.