How to Read Pre OBD2 Codes Without Scanner (GM)

Before the standardization of On-Board Diagnostics with OBD2 in 1996, General Motors (GM) vehicles relied on a system often referred to as “pre-OBD2” for engine diagnostics. While these earlier systems lack the universal OBD2 connector and standardized codes, there are still ways to retrieve diagnostic information without a dedicated scanner. This article will guide you through the process of reading pre-OBD2 codes on your GM vehicle, empowering you to troubleshoot issues and potentially save on diagnostic fees.

Understanding Pre-OBD2 Systems in GM Vehicles

Unlike OBD2, which utilizes a standardized 16-pin connector and digital codes, pre-OBD2 systems in GM vehicles varied significantly between models and even model years. Some systems employed flashing “Check Engine” lights with specific patterns to indicate fault codes, while others required jumper wires to connect specific terminals in the Assembly Line Diagnostic Link (ALDL) connector. The location of the ALDL connector also varies depending on the vehicle model, often found under the dashboard or in the engine bay.

Reading Pre-OBD2 Codes on GM Vehicles

While a dedicated code reader is the easiest way to read pre-OBD2 codes, you can retrieve the codes yourself using a few simple steps:

1. Locate the ALDL Connector:
The ALDL connector is typically a rectangular connector with multiple pins. Consult your vehicle’s service manual or online resources to determine its exact location.

GM ALDL Connector Location

2. Identify the Diagnostic Terminals:
Not all pins in the ALDL connector are used for diagnostics. You’ll need to identify specific terminals labeled “A” and “B” for most GM models.

3. Connect a Jumper Wire:
Use a jumper wire or a paperclip to connect terminals “A” and “B” on the ALDL connector.

4. Turn the Ignition On:
Turn the ignition to the “On” position without starting the engine.

5. Observe the “Check Engine” Light:
The “Check Engine” light will begin to flash in a specific sequence. Note down the number of flashes and pauses.

GM Check Engine Light Flashing Codes

6. Decode the Flashing Sequences:
Each sequence of flashes represents a specific digit in the code. For instance, one flash might represent “1,” two flashes “2,” and so on. A longer pause usually separates individual digits within a code.

7. Consult a Code Chart:
Once you have the complete code, refer to a GM pre-OBD2 code chart specific to your vehicle’s model year and engine type. These charts are readily available online and in service manuals.

Common Pre-OBD2 Codes on GM Vehicles

While the specific codes vary between models, some common pre-OBD2 codes on GM vehicles include:

• Code 12: Normal diagnostic mode indication (no fault detected).
• Code 13: Oxygen sensor circuit issue.
• Code 22: Coolant temperature sensor issue.
• Code 33: Manifold Absolute Pressure (MAP) sensor issue.
• Code 44: Oxygen sensor lean condition.

“Understanding the nuances of pre-OBD2 systems can be challenging,” says John Smith, Senior Automotive Technician at XYZ Auto Repair. “However, with a little patience and the right resources, it’s entirely possible for vehicle owners to diagnose basic engine issues themselves.”

Limitations and Precautions

It’s important to note that reading pre-OBD2 codes without a scanner has limitations:

• Code Availability: Not all pre-OBD2 systems store codes for every engine component.
• Code Specificity: Pre-OBD2 codes are generally less specific than OBD2 codes, often indicating a general area of concern rather than a specific fault.
• System Variations: The process of retrieving codes can vary significantly between GM models.

Always disconnect the negative battery terminal before working on any electrical components in your vehicle. If you are uncomfortable working on your car, consult a qualified mechanic.

Conclusion

While pre-OBD2 systems may seem archaic compared to the standardized OBD2, understanding how to read codes without a scanner can be invaluable for owners of older GM vehicles. By following the steps outlined in this article and consulting the appropriate resources, you can gain valuable insights into your vehicle’s engine health and potentially address minor issues before they escalate.

Remember, while reading pre-OBD2 codes can provide a starting point for troubleshooting, it’s crucial to consult a qualified mechanic for accurate diagnosis and repair, especially for complex issues.