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Reading the Diagnostic Trouble Codes

If you have noticed that the Check Engine Light on the dashboard of your car is illuminated, it probably means that your car is experiencing a problem that relates to the emission system. On vehicles that are equipped with an OBD II diagnostic system, if one or more diagnostic trouble codes are triggered, then the Check Engine Light will be turned on. This is to ensure that you can find out the problem by reading the trouble code using a code reader or a scan tool. You can usually do this by connecting the device (normally located under the dashboard) to the OBD II diagnostic connector. The device will show you the specific trouble codes that trigger the Check Engine Light on the dashboard.

A proper device such as a scan tool is required to read the trouble codes. If your vehicle is produced before 1996, you might need to use a scan tool that is suitable for OBD I diagnostic system. Different vehicles might also have a different diagnostic connector for the system, which will mean that you will need a proper adapter for the connectors as they are not standardized. But, newer vehicles with OBD II diagnostic systems will usually have similar connectors and you will not be required to use an adapter. The differences may come from the software and hardware that are used on different brands and models of the car.

OBD2 Scanner

Reading the Diagnostic Trouble Codes

  1. Find the OBD II diagnostic connector under the dashboard. You can usually notice the connector by looking at the 16-pin that connects the system. Note: You may need to take apart the knee bolster panel on the vehicle to get access to the connector. Some vehicles might place the connector in the center console. You can also check the correct location of the connector by reading the vehicle’s manual.
  2. Connect the scan tool or code reader to the diagnostic connector.
  3. Turn on the vehicle ignition system without starting the engine. This will initiate the process required to connect the scan tool to the engine’s computer.
  4. Find the button that says READ CODES on the scan tool. Different scan tools might have different options to read the codes. Note: On some occasions, you will need to insert the information about your car into the scan tool as it will not automatically recognize the specific model of your vehicle.
  5. The diagnostic fault codes should be displayed on the monitor of your scan tool. Make sure to write down the codes so you will not forget them later.
  6. You can proceed by erasing the diagnostic fault codes by pressing the button that says CLEAR CODES on the scan tool menu.
  7. VERY IMPORTANT: The diagnostic fault codes will not indicate the type of components that you have to replace. Instead, it will only tell you the problem that triggers a certain sensor circuit on the engine system. You will still need to get a proper inspection to find out the cause of the problems and replace the components that are damaged or faulty.
  8. Your Check Engine Light will still be illuminated even after you clear the diagnostic fault codes. The Check Engine Light will not behave normally until you address the real problem that is causing a fault code on the engine.

Check Engine Light On

Understanding the Check Engine Light

The Check Engine Light, or also known as the Malfunction Indicator Lamp, will be illuminated when the engine computer detects that there is something abnormal about the system that can affect the emission output. The light may be turned on and off again automatically by the system depending on how the vehicle is configured or the intensity of the problem. The light might also flash continuously when the vehicle is being operated. If the vehicle is experiencing frequent problems, the light may be turned on when the problem is occurring, and it will be off when the problem goes away. Some engine problems may also cause the light to be on continuously until the problem is addressed and completely fixed.

Some drivers say that the illumination of the Check Engine Light can be annoying as it does not tell the driver about the specific problems from the engine. The Check Engine Light can be turned on both by serious or minor problems. Drivers can only find out about the specific problems if they connect the diagnostic system with a scan tool to find out about the trouble codes.

On most occasions, you can usually keep driving the vehicle even if the light is turned on. But, you have to make sure that the engine is running normally and you don’t notice any warning lights being turned on. Still, you will need to inspect the engine as soon as possible to find out about the real cause of the problem. Another thing to consider is that your vehicle will have difficulty passing the vehicle emission testing if the Check Engine Light is on or a diagnostic trouble code has not been cleared from the system.

How to Clear the Trouble Codes

If your vehicle is equipped with the older generation of the OBD II onboard diagnostic system, you can simply erase the fault codes by disconnecting the computer system from the vehicle power source, such as the battery. The system’s memory will be wiped out due to the loss of voltage which will lead to the CHECK ENGINE LIGHT being automatically turned off. But, this is usually just a temporary fix as the light will be turned on again when the original problem reoccurs and the trouble codes get stored in the memory once again.

If your vehicle uses a newer diagnostic system, the computer usually uses a nonvolatile memory to store the fault codes. This means that the trouble codes will not be lost even if the battery is disconnected. You can usually clearly remaining code using a compatible scan tool. Disconnecting the diagnostic system from the power source is also not recommended as it can cause other problems in the diagnostic system, such as the loss of radio presets and climate control settings. You will also need to restart the adjustments made by the system that is usually used to compensate for personal driving habits to reduce engine wear. The loss of electricity to the diagnostic system might also require you to put the engine computer through a special learning procedure to return the proper transmission operation on the vehicle.

The Procedure to Set the Trouble Codes by OBD II

Before the OBD II diagnostic system is widely available for modern vehicles, the system can only detect gross failures that occur on certain sensors or circuits. The older generation of the diagnostic systems did not have the ability to detect other problems, such as the cause of frequent misfiring, the proper function of the catalytic converter, or the leakage of fuel vapors from the engine. The Newer OBD II system added the capability to monitor those aspects so the system can detect emissions problems when they occur.

A similar procedure on the OBD II diagnostic system is still used in which it will illuminate the Check Engine Light to notify the driver that there is a problem with the engine. The system will also store the fault codes to the memory that can be inspected to find the specific cause of the problem. The additional feature is the capability of the system to track the engine problems as they occur and develop over time.

If the system detects that the problem on the emission system is causing the vehicle to produce hydrocarbon emissions that exceed 1.5 times the federal limit, the diagnostic system will trigger the Check Engine Light to be turned on. The light was still be illuminated even if the driver cannot notice any drivability problem on the vehicle.

One of the worthiest upgrades of the OBD II diagnostic system is its ability to detect problems that can cause frequent misfiring on the engine. The older diagnostic system did not have the capability to do that. Therefore, drivers frequently did not know the proper condition of the engine performance. While different vehicle manufacturers use different strategies to detect misfiring problems, most of the systems usually employ the crankshaft position sensor to detect the speed of the crankshaft and output the information as a signal. Misfiring in the engine may lead to the improper rotational velocity of the crankshaft. OBD II system usually detects the position of the crank to know the proper firing sequence of each cylinder. This means that the system will be able to specify the cylinder that causes misfiring. It will then turn on the Check Engine Light and track the cylinder that is causing frequent misfiring.

How to Read the Trouble Codes

The diagnostic system will store the fault codes that can be associated with a cylinder that causes misfiring with a specific number, such as P030X where “X” is the specific cylinder number that causes the problem. For instance, if the code P0302 is being shown, then the problem comes from cylinder number two. But, the system will not tell you the real cause of the misfiring. Another diagnostic testing will be required to figure out the cause. The condition might be the result of a failed spark plug, damaged wiring, a broken ignition coil, a broken fuel injector, a leak on the head gasket, or other related conditions.

Some OBD II systems used on certain vehicles will also disable certain cylinders if it causes frequent misfiring in the engine. The system does this to protect the catalytic converter. The diagnostic system will turn off the fuel injector mechanism for the certain cylinder to prevent the exhaust to be drowned with unburned fuel. This is important because raw fuel can create excessive overheat in the converter which will damage the component.

The OBD II system also has the capability to monitor the catalytic converter by getting a signal from the second oxygen sensor. It does this by comparing the data received from the upstream and downstream oxygen sensors to find out the proper operation of the converter. By reading the converter efficiency level, the OBD II system will be able to decide if it needs to trigger the Check Engine Light.

The system can also be used to detect fuel vapor leaks from the engine. Vapor leaks can come from the EVAP plumbing or charcoal canister. The diagnostic system can also detect if the gas cap is loose or missing.

Some problems with the electronic transmission system or the vehicle air conditioning system can also be detected using the OBD II system.

Differences Between Fault Codes

The law requires that all OBD II systems used on most vehicles should produce generic codes with similar codes as well as a similar connector for the diagnostic system using the 16-pin connector. This means that the fault code P0301 on different vehicles from different manufacturers should point out the same misfiring problem. But, some manufacturers usually choose to add unique codes that can provide a more detailed information regarding the fault code. The enhanced codes may also include problems that happened outside the engine system. For instance, some fault codes are related to the airbags, HVAC, ABS, and other components of the vehicle.

Using a basic scan tool can usually detect the generic codes produced by the OBD II diagnostic system. A scan tool designed for older diagnostic systems may not be compatible with the newer OBD II system. Some scan tools for the OBD II system should also be CAN-compliant. If you want to get the correct reading, you will need to ensure that the scan tool that you use is compatible with your vehicle’s diagnostic system.

Some cheap code readers also do not have the capability to read fault codes that are not generic codes. Some may not be able to read specific P1 fault codes from the vehicle manufacturer. Scan tools that can read P0 in P1 codes are usually sold in North American market, but customers in Asian and European markets may have difficulties finding such scan tools.

Be Careful with False Codes!

Modern vehicles are usually equipped with a sensitive OBD II diagnostic system. The sensitive component can even set misfiring code even if the engine does not have frequent misfiring. This condition can lead to the system generating false misfire readings if the vehicle is being operated under certain conditions. For instance, vehicles that are frequently being driven on extreme terrains may produce unstable variations in the crank speed that can be translated to misfiring by the diagnostic system. Some system usually detects the rough terrain condition and lower the level of sensitivity. Meanwhile, some systems will instead detect the firing voltage of certain spark plugs to detect misfiring.

A misfire code can also be triggered if the misfiring is not connected to a particular cylinder. For instance, if the system is showing a fault code P0300, it means that the system is detecting misfiring from different cylinders. This condition may be the result of a lean fuel mixture or a vacuum leak inside the intake manifold. Some more specific misfiring codes should be thoroughly inspected by a technician to find the real cause of the problem.

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