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Reading Fault Codes
 
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Pelican Technical Article:

Reading Fault Codes

Nick Czerula

Time:

2 hours2 hrs

Tab:

$0

Talent:

**

Tools:

Scan tool, Code reader

Applicable Models:

BMW Z3 (1996-02)

Hot Tip:

Record fault code and description before clearing

Performance Gain:

Repair fault codes

Complementary Modification:

Monitor scan tool data stream to help diagnose fault codes

The impetus to control motor vehicle emissions, driven by the air quality needs of major metropolitan areas, has lead to 5 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 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 the engine operations allow two benefits in addition to a precise air fuel mixture and, therefore, lower 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, on the instrument cluster. The stored faults can be viewed later for diagnosis and repair.

Diagnosis of modern vehicle performance faults start 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 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 diagnostic and repair instructions on 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.
Figure 1

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. This scan tool shows two fault codes stored in our vehicle, one for low voltage and the other for the coolant temperature sensor.

Early BMW Z3 models have two data link connectors (DLCs) to connect a scan tool to.
Figure 2

Early BMW Z3 models have two data link connectors (DLCs) to connect a scan tool to. If you are using a generic scan tool, you can connect to the 16 pin OBD II connector (green arrow) at the lower right side of the dashboard, on center console next to radio (passenger side). Be sure to reinstall the plastic cover (yellow arrow) when complete. On some models, this can mess up communication on the 20 pin connector if left off. Late BMW Z3 models may only have a 16 pin OBD II connector. Once they moved to this style of plug, you get full access to all the systems when using an advanced BMW scan tool.

If you are using a BMW scan tool or a Peake code reader, you can connect to the 20 pin connector (green arrow) on the right side of the engine compartment.
Figure 3

If you are using a BMW scan tool or a Peake code reader, you can connect to the 20 pin connector (green arrow) on the right side of the engine compartment.

Connecting a Scan Tool to 16 pin OBD II connector - Working at right side of dashboard, open OBD II connector access cover.
Figure 4

Connecting a Scan Tool to 16 pin OBD II connector - Working at right side of dashboard, open OBD II connector access cover. Slide cover up in direction of green arrow, then remove from center console and allow to hang by attached tether.

Remove OBD II connector plastic cover (yellow arrow), then connect scan tool to 16 pin plug (green arrow).
Figure 5

Remove OBD II connector plastic cover (yellow arrow), then connect scan tool to 16 pin plug (green arrow).

Plug scan tool into OBD II connector.
Figure 6

Plug scan tool into OBD II connector. Follow directions supplied with scan tool to interrogate ECM fault memory. Read fault codes, diagnose problem, then clear fault code when complete.

Connecting a Scan Tool to 20 pin BMW connector - Working at right side of engine compartment, unscrew 20 pin BMW connector lid.
Figure 7

Connecting a Scan Tool to 20 pin BMW connector - Working at right side of engine compartment, unscrew 20 pin BMW connector lid. Plug scan tool into 20 pin BMW connector (green arrow).

Follow directions supplied with scan tool to interrogate ECM fault memory.
Figure 8

Follow directions supplied with scan tool to interrogate ECM fault memory. Read fault codes, diagnose problem, then clear fault code when complete. You can see that when using the 20 pin BMW connector, you have access to all systems, not always the case the 16 pin OBD II connector.

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Comments and Suggestions:
Josh49 Comments: Hi Nick,
I meant the 20 pin port of course. You were right, it does open anticlockwise but you have to grip the sides and not the top. Many thanks for your help.
November 5, 2015
  Followup from the Pelican Staff: Correct, the knurled part of cover has to be rotated. - Nick at Pelican Parts  
Josh49 Comments: I cannot get this connector open. Looking at the photo it looks to me as though it is a left hand thread. Is this so?
Thanks
November 3, 2015
  Followup from the Pelican Staff: What connector? The 20 pin port? It unscrews counterclockwise. - Nick at Pelican Parts  

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