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How to Read Diagnostic Trouble Codes

Copyright AA1Car

Check Engine Light On? That means the onboard diagnostic system on your vehicle has detected a problem that could increase emissions. When this happens, the OBD II system sets one or more diagnostic trouble codes that correspond to the problem detected. To read the codes, you will have to plug a scan tool or code reader into the 16-pin OBD II diagnostic connector, which is usually located under the dash near the steering column. The tool will then display the code or codes that have turned on the Check Engine Light.

To read codes, you need the proper scan tool. On older (pre-1996) vehicles, an OBD I scan tool is required. Since connectors were not standardized, the scan tool must have the proper adapter for the vehicle's diagnostic connector (since they were all different). On 1996 and newer vehicles is OBD II, the connectors are all the same - but the software (and hardware) that is required to read the codes can vary depending on the year, make and model of your vehicle.


1. Locate the 16-pin OBD II diagnostic connector (usually under the dash near the steering column). Note: On some vehicles, it may be necessary to remove a knee bolster panel or other panel to find the connector. On some the connector may be located in the center console or someplace else. If you cannot locate the OBD II diagnostic connector, refer to your vehicle owners manual.

2. Plug in your code reader or scan tool.

3. Turn the ignition ON, but do not start the engine. This is usually necessary so the scan tool can communicate with your car's computer.

4. Depending on the scan tool you are using, push the READ CODES button or select the READ CODES option on the tool menu. Note: some scan tools do not automatically recognize the year, make and mode of your vehicle. You will first have to enter this information before the scan tool will read any fault codes.

5. Your scan tool should display any diagnostic fault codes that are present in numerical order. WRITE DOWN THE CODES! This is important for later reference. If your tool does not also display the definition of the code, Click Here to look up the definition of the code(s).

6. You can no safety clear (erase) the codes by pressing the CLEAR CODES button or choosing the erase code option on the scan tool menu.

7. VERY IMPORTANT: Most codes DO NOT tell you what part to replace,only that a problem has occurred in a particular sensor circuit or system. Further diagnostics will usually be necessary to diagnose the faulty part that needs to be replaced to fix your problem.

8. Clearing the codes does NOT make the problem go away. If the problem is still present, sooner or later the Check Engine Light will come back on the the same code(s) will reset.

For more information about different types of scan tools, Click Here


The "Malfunction Indicator Lamp" (or Check Engine Light as most people call it) is supposed to come on when a problem occurs in the engine control system that affects emissions. Depending on how the system is configured and the nature of the problem, the lamp may come on and go off, remain on continuously or flash. Some types of intermittent problems will make the lamp come on only while the fault is occurring. When the problem goes away, the lamp goes off. Other types of problems will turn the light on, and it will remain on until the fault is diagnosed and repaired.

The Check Engine lamp has proven to be a great annoyance to many motorists (as well as your professional customers) because it doesn't tell you anything about the nature of the problem. The problem could be something serious or relatively minor. There is no way to know until you hook up a scan tool and read out the code(s) that caused the light to come on.

If the engine seems to be running normally, and no other warning lights are on, you can probably ignore the light and keep driving. But you should read the codes as soon as possible to find out what is going on. Also, if you live in an area that requires vehicle emissions testing, you vehicle will NOT pass an emissions test if the Check Engine Light is on or there are any live trouble codes in memory.

Clearing Trouble Codes

In most of the first generation onboard diagnostic systems prior to OBD II, disconnecting the computer's power source or disconnecting a battery cable could erase fault codes. The loss of voltage wiped out the computer's temporary memory causing the Check Engine light to magically go out. But as soon as the original problem reoccurred, the code(s) would be reset and the light would come back on.

In most newer computer systems, fault codes are stored in a "nonvolatile" memory that is not lost if the battery is disconnected. The codes remain intact until they are cleared using a scan tool (which few motorists own). What's more, disconnecting the battery or computer's power supply can have undesirable consequences because it causes the loss of electronic presets in the radio and climate control system, as well as the engine computer's "learned" memory - the adjustments that are made over time to compensate for engine wear and driving habits. On some vehicles where the computer also regulates the electronic transmission, the computer may have to be put through a special learning procedure to relearn the proper operation of the transmission if power has been lost!


Prior to OBD II, fault detection was mostly limited to "gross failures" within individual circuits or sensors. The first generation systems couldn't detect engine misfire, how well the catalytic converter was functioning or whether a vehicle was leaking fuel vapors into the atmosphere. OBD II changed all of that by adding the ability to monitor these things so emission problems can be detected as they develop.

OBD II still uses the Check Engine lamp to alert the driver when a fault occurs, and it still stores fault codes that correspond to specific kinds of problems, but it adds the unique ability to track problems as they develop and to capture a snapshot of what's going on when a problem occurs.

Almost any emission problem that causes hydrocarbon emissions to exceed 1.5 times the federal limit can cause the Check Engine light to come on with OBD II - even if there is no noticeable drivability problem accompanying the emission problem.

The most powerful (and controversial) feature of OBD II is its ability to detect engine misfire. First generation OBD systems couldn't do that directly so there was no way to know if the engine was performing properly or not. OBD II misfire detection strategies vary somewhat from one vehicle manufacturer to another, but most currently use the input from the crankshaft position sensor to monitor changes in crankshaft speed. A single misfire will cause a slight variation in the rotational velocity of the crank. By knowing the position of the crank and which cylinder is supposed to be firing, the OBD II system can correlate each misfire that occurs with a specific cylinder. The misfires are tracked and tabulated, and if a pattern occurs it can set a misfire code and turn on the Check Engine light.

For a detailed look at the operating parameters that can set various fault codes, Click Here to view a PDF file on GM 4.6L diagnostic parameters.

scan tool fault codes


A misfire that occurs in a given cylinder will set a P030X code where "X" will be the number of the cylinder that is misfiring. For example, a P0302 code would tell you cylinder number two is misfiring. But here's the important point: The code does not tell the technician why the cylinder is misfiring. He has to figure that out for himself by performing other diagnostic tests. The misfire might be due to a fouled spark plug, a bad plug wire, a defective ignition coil in a DIS system, a clogged or dead fuel injector or a loss of compression due to a leaky exhaust valve, leaky head gasket or worn cam lobe.

On some vehicles, the OBD II system itself will disable a cylinder if it detects a high enough rate of misfire. This is done to protect the catalytic converter. By shutting off the cylinder's fuel injector, the OBD II system prevents unburned fuel from passing through the cylinder and entering the exhaust. Raw fuel in the exhaust is bad news because it makes the converter overheat, and if it gets too hot it can suffer damage.

What else does OBD II add to the equation? It also monitors the operation of the catalytic converter with a second oxygen sensor on the tailpipe side of the converter. By comparing upstream and downstream O2 sensor readings, it can determine how well the converter is doing its job. If converter efficiency drops below a certain threshold, the OBD II system will set a code and turn on the Check Engine light.

OBD II can also detect fuel vapor leaks (evaporative emissions) in the charcoal canister, evap plumbing or fuel tank by pressurizing or pulling a vacuum on the fuel system. It can even detect a loose or missing gas cap.

In addition, OBD II can also generate codes for various electronic transmission problems and even air condition failures such as a compressor failure.


The diagnostic codes that are required by law on all OBD II systems are "generic" in the sense that all vehicle manufacturers use the same common code list and the same 16-pin diagnostic connector. Thus, a P0301 misfire code on a Ford means the same thing on a Chevy, Chrysler, Toyota or Mercedes. But each vehicle manufacturers also have the freedom to add their own "enhanced" codes to provide even more detailed information about various faults. Enhanced codes also cover non-emission related failures that occur outside the engine control system. These include ABS codes, HVAC codes, airbag codes and other body and electrical codes.

The "generic" codes that are common to all vehicle manufacturers can be accessed using any basic scan tool that is OBD II compliant. An older scan tool desined for OBD I vehicles cannot read OBD II fault codes unless is has been updated with new software. Even then, many ofthese tools do not have the proper hardware for reading OBD II codes. The same applies to OBD II scan tools that are not CAN-compliant. Around 2006, most vehicles got controller area network (CAN) electrical systems that require different scan tool hardware and software to read. So make sure the scan tool you are attempting to use to read diagnostic codes is the correct one for your vehicle.

Another problem you may encounter with inexpensive code readers and scan tools is that they will only read generic PO fault codes, or that they cannot read some or all of the manufacturer specific P1 fault codes. Many scan tools sold in the North American market can read Ford, GM and Chrysler P0 and P1 codes, but have little or no coverage for P1 codes on Asian and European makes.

Today's OBD II systems are so sensitive to misfires that they will set a misfire code if they detect as few as five misfires in 200 engine revolutions! Unfortunately, this high level of sensitivity can sometimes generate false misfire readings under certain operating conditions. Driving on an extremely rough road, for example, can produce the same kind of variations in crank speed that appear to be misfires to the OBD II monitor. Some newer OBD II systems compensate for rough road operation by reducing the level of misfire sensitivity. Others use a different method to detect misfires. Instead of monitoring crankshaft speed, the system watches the firing voltage of each spark plug to detect problems (a lean misfire typically causes a large jump in the firing voltage while a shorted or fouled plug causes a drop in the firing voltage).

Random misfires that are not isolated to a particular cylinder will also set a misfire code. A P0300 code means the OBD II system has detected multiple misfires in multiple cylinders. The underlying cause in most cases is a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor. As with the individual cylinder misfire codes, the technician still has to figure out what's causing the problem and then fix it.

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