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Reading Vehicle Fault Codes

Pelican Technical Article:

Reading Vehicle Fault Codes

Nick Czerula


1 hours1 hrs






Scan tool, Code reader

Applicable Models:

BMW Z4 Convertible (2003-06)
BMW Z4 Coupe (2006)

Parts Required:

Code reader

Hot Tip:

Start with a fully charged battery

Performance Gain:

Repair fault codes

Complementary Modification:

Do not clear fault codes until you think you have repaired the problem

The impetus to control motor vehicle emissions, driven by the air quality needs of major metropolitan areas, has led to five decades of engineering innovation in the design of vehicle propulsion technologies. Because California cities (particularly Los Angeles) were hardest hit by the proliferation of vehicles and their emissions, starting in 1961 the California Air Resources Board (CARB) has been at the forefront of setting engineering standards for vehicles sold, first in California, then in the United States and now for the world automotive industry.

Modern vehicle engine operations and emissions (including evaporative losses from the fuel tank) are extensively controlled via a digital microprocessor, the computer known as the engine control module or ECM. Feedback systems, relying on sensors, allow the ECM to precisely control fuel / air mixture, spark timing and other functions. Of the many sensors on the engine and the vehicle, the primary ones are oxygen sensor(s), which monitor the combustion efficiency of the engine, and knock sensor(s), which "listen" for engine knock or pinging. The input signals from these sensors are used by the ECM to adjust fuel mixture and adjust spark timing.

Computer control of engine operations allows two benefits in addition to a precise air-fuel mixture and, therefore, lowers emissions. The computers can be programmed to detect faults (diagnostic trouble codes or DTCs) in their own operations, and the ECM can store this information and illuminate a malfunction indicator light (MIL), often called the Check Engine light or malfunction indicator lamp (MIL), on the instrument cluster. The stored faults can be viewed later for diagnosis and repair.

Diagnosis of modern vehicle performance faults starts with gaining access to DTCs stored in the ECM. Start by looking for the 16-pin OBD II plug (diagnostic link connector or DLC) under the dashboard. By law, it is always located within reach of the driver.

Once the diagnostic scan tool is connected to the DLC, follow the instructions on the scan tool screen. These usually include:

  • Select diagnosis program.
  • Select correct vehicle (scan tool should find this automatically).
  • Interrogate and record DTCs (fault codes) present in ECM memory.

In many cases, it is best to clear DTCs. Then drive the vehicle and retest. Follow the diagnostic and repair instructions on the scan tool screen. DTCs for the vehicle power train, as standardized by the American Society of Engineers (ASE), begin with a P and are followed by 4 digits. (They are also referred to as P-codes.) During testing, the scan tool displays each P-code and a brief description of the fault. There are different kinds of fault indicated by DTCs:

  • Plausibility. If a sensor's output value is outside the expected range, the signal from the sensor is considered not plausible, and a fault is set.
  • Power, ground, continuity. If power or ground is missing or continuity is lacking in a particular circuit or system, a fault is set.
  • Fault in sensor or module. If a sensor or module tests defective, a fault is set.

You should note that even though the scan tool displays specific DTCs, additional diagnosis is advisable. For example, if a DTC indicates a non-functional oxygen sensor heater, the simple explanation may be a blown fuse. In other words, use common sense, and do not ignore the obvious when diagnosing problems.

BMW Z4 models have one of the most complicated BMW electrical systems to date. When addressing fault codes it is important to keep this in mind. There are many control modules that now share the control of a single function. For example, the general module controls the locks and power windows. BMW calls this type of system distributed functions. This is where a more advanced BMW scan tool is needed to completely diagnose some fault codes your BMW may set. Do not be discouraged from attempting to diagnose fault codes on your own. Just know there will be limits to how far you can go. As stated above, the engine management system monitors emission components and will set a fault code in the DME when a fault is present. These are the codes you will have to work with the most. Treat them as you normally would and inspect the component and system the fault code relates to. If you find a code will not clear or points to a system your scan tool cannot access, you may need a more advanced scan tool. Know your scan tool and the limits it is designed with, and use all available resources to help during your diagnosis.

When considering a scan tool, think about what you want to diagnose. A generic code reader or less advanced scan tool may only display OBD II related engine fault codes. A more advanced scan tool will display fault codes from the entire vehicle electrical system, as well as perform adaptations, coding and programming.

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Connecting a Scan Tool:
Working at the driver side kick panel, locate the OBD 16 pin plug cover (red arrow).
Figure 1

Working at the driver side kick panel, locate the OBD 16 pin plug cover (red arrow).

Fold open the OBD II connector access door (red arrow).
Figure 2

Fold open the OBD II connector access door (red arrow).

Plug your scan tool into the OBD II connector (red arrow).
Figure 3

Plug your scan tool into the OBD II connector (red arrow).

Follow the directions supplied with the scan tool to interrogate the vehicle fault memory.
Figure 4

Follow the directions supplied with the scan tool to interrogate the vehicle fault memory. Read the fault code and diagnose the problem. Then clear the fault code when complete. Keep in mind when using a standard or generic fault code reader (green arrow) you may not display all systems, fault codes or fault code numbers. If deeper diagnosis or interrogation is needed, I suggest using an advanced scan tool (red arrow) such as an Autologic. If you do not have access to an advanced tool, you can ask a local work shop to read and print the fault memory for you. If you have the ability, perform a full vehicle scan (of all the control modules). This will return fault codes for the entire electrical system, better helping you diagnose your electrical issues.

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Page last updated: Tue 2/20/2018 03:06:10 AM