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Deciphering the OBD-II Fault Code

Technically Speaking

James Berry Oct 28, 2016
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I was recently at a seminar going over some diagnostics for some of the problematic and reoccurring fault codes. The general discussion led us in an interesting direction about the nature of fault codes so I thought I would share some of this information with you.

Now for the questions:

Do the letters and numbers of fault codes have a meaning or are they just a random list some engineer made up?

I have seen the check engine light that stays on constantly, and I have also seen the light flash the entire time the car is running. Why the difference?

What does each pin of the 16-pin data link connector control?

Fault Codes

Let’s start with a breakdown of what each digit of the OBD-II fault code means. This will help you understand the code itself.

The first character identifies the system related to the trouble code. This is important because it will let you know what system has a problem.

P = Powertrain
B = Body
C = Chassis
U = Undefined/ Network communications

Fault Codes 2/3

The second character identifies whether the fault code is a generic code or a manufacturer specific code. A generic code is the same on all OBD-II equipped vehicles no matter the manufacturer. A manufacturer specific code is one that is used for only that manufacturer.

Powertrain (Example: P0)

0 = Generic OBD Code, SAE Defined Code
1 = Manufacturer Specific Code
2 = Manufactured or Generic Specific Code, depending on the model
3 = 3000-3399, Manufacturer Defined Code; 3400-3999, SAE Defined Code (Generic OBD)

Body Codes and Others (Example: B1, C0, or U2)

0 = Generic OBD Code, SAE defined
1 = Manufacturer Specific Code
2 = Manufactured Specific Code
3 = For Future Allocation

The third character denotes the system that relates to the code (Example, P01 = Fuel or Emission Management). The trouble code is hexadecimal and therefore the third, fourth, and fifth characters can be either a number (0-9) or a letter (A-F).

1 = Fuel or Emission Management
2 = Injector Circuit (Fuel or Air)
3 = Ignition or Misfire
4 = Emission Control
5 = Vehicle Speed & Idle Control
6 = Computer & Output Circuit
7 = Transmission
8 = Transmission
9 = Transmission

A = Hybrid Only
B = Hybrid Only
C = Hybrid Only
D = For Future Allocation
E = For Future Allocation
F = For Future Allocation

The fourth and fifth digits relate to a particular problem where the trouble exists. Let’s use the fault code P0303 as an example. The first letter P indicates a powertrain problem. The 0 indicates that it is a generic code. The number 3 indicates that the failure is in the misfire monitoring system. The last two digits, 03, indicate that the number three cylinder has detected a misfire. In our example you can see that the last two digits of the code will correspond to the number of the misfiring cylinder (fault codes for individual cylinders of a V-8: P0301, P0302, P0303, P0304, P0305, P0306, P0307, and P0308).

Check Engine Light

The check engine light on your dash is designed to show you three different types of problems.

1. A check engine light that occasionally flashes shows an intermittent malfunction.
2. A check engine light that stays on constantly indicates that the problem is hard failure, affecting the emissions output, driveability or safety of the vehicle.
3. A check engine light that is flashing while the car is running is a sign of a serious problem, which can cause damage to the engine. This type of warning should be taken seriously and the car should not be driven in most cases.

16 Pin Data Collector 3/3

Configuration of the 16-Pin Data Link Connector
Terminal Designation
PIN 1 Manufacturer Specific
PIN-2 SAE J1850 Bus
PIN-3 Manufacturer Specific
PIN-4 Chassis Ground
PIN-5 Signal Ground
PIN-6 CAN Data Bus, High ISO 15765-4
PIN-7 K-Line – ISO 9141-2/ISO 14230-4
PIN-8 Manufacturer Specific
PIN-9 Manufacturer Specific
PIN-10 SAE J1850 Bus
PIN-11 Manufacturer Specific
PIN-12 Manufacturer Specific
PIN-13 Manufacturer Specific
PIN-14 CAN Data Bus, Low ISO 15765-4
PIN-15 L-Line – ISO 9141-2/ISO 14230-4
PIN-16 Battery Impute Positive

Note: some of these cavities may be empty and indicate that they are not used with that vehicle.

Do not rely on fault codes to tell you what component to replace if the fault code reports a sensor fault. Replacement of the sensor may not resolve the underlying problem. The fault could be caused by something else in the systems that the sensor is monitoring. For example, oxygen sensor lean or rich faults are likely reporting fuel mixture problems.

I hope this helps in understanding how the OBD-II system is set up and in diagnosing any problems you may have with your vehicle in the future. I love all of the questions I get from everyone. You guys keep me on my toes.

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