Decoding 91 GMC OBD2 Codes: Your Guide to Understanding OBD-I Diagnostics

While you might be searching for “91 Gmc Obd2 Codes,” it’s important to clarify that vehicles from 1991, including GMC models, typically utilize the OBD-I (On-Board Diagnostics I) system, not OBD-II. Understanding this distinction is the first step in effectively diagnosing your 1991 GMC truck or vehicle. This guide will walk you through the process of reading OBD-I trouble codes on your 91 GMC, helping you pinpoint potential issues and take informed steps towards repair.

Understanding OBD-I System in Your 1991 GMC

The OBD-I system, while less sophisticated than the later OBD-II, still provides valuable insights into your vehicle’s health. For 1991 GMC trucks and similar models, accessing these diagnostic codes is surprisingly straightforward and doesn’t require any specialized tools. The system relies on your vehicle’s “Service Engine Soon” light to communicate error codes through a series of flashes.

This article will serve as your expert guide to navigating the OBD-I system in your 1991 GMC, ensuring you can effectively retrieve and interpret these codes. We’ll cover the simple process of accessing the codes and provide a comprehensive list of common codes specific to gasoline engines of that era.

Step-by-Step Guide to Checking OBD-I Codes on Your 91 GMC

Retrieving trouble codes from your 1991 GMC’s OBD-I system is a simple process that you can perform yourself. All you need is a paper clip or a short piece of wire to act as a jumper. Here’s how to do it:

Locate the DLC Connector: The Diagnostic Link Connector (DLC), also sometimes referred to as the Assembly Line Diagnostic Link (ALDL) connector, is the access point for retrieving codes. On 1991 GMC trucks, it’s typically located under the driver’s side dashboard.
Identify Terminals A and B: Once you’ve located the DLC connector, you need to find terminals A and B. These are usually labeled, but if not, the diagram below illustrates their position – they are generally the top right terminals on many common DLC connectors of this era.

This image shows a typical OBD1 ALDL connector, highlighting terminals A and B which are essential for initiating the diagnostic code retrieval process on your 1991 GMC.

Jumper Terminals A and B: With the ignition switch turned off, use your paper clip or wire to create a jumper wire connecting terminal A to terminal B on the DLC connector.
Turn Ignition On (Engine Off): Turn the ignition switch to the “ON” position, but do not start the engine.
Observe the “Service Engine Soon” Light: Watch the “Service Engine Soon” light on your dashboard. It will begin to flash a series of codes.
Decoding the Flashes: The codes are communicated through flashes, pauses, and flash sequences. Code 12 is the starting point and indicates the system is working correctly. It is represented by flash, pause, flash flash (one flash, a pause, then two flashes), signifying “1” and “2”.
Code Repetition and Sequence: Each code will be displayed three times before moving to the next code in numerical order. The entire sequence of codes will repeat continuously until you turn the ignition off or remove the jumper. Code 12 will also repeat in the sequence, which is normal and not indicative of a problem itself.

1991 GMC OBD-I Code List for Gasoline Engines

It’s crucial to remember that an OBD-I code is a starting point for diagnosis, not a definitive answer. A code indicates that a sensor or system is reporting data outside of its expected range. The root cause might be the sensor itself, or it could be a related issue affecting the sensor’s readings. Effective troubleshooting involves using the code as a guide to investigate further.

Here is a list of common OBD-I codes relevant to gasoline engines in 1991 GMC vehicles. Note that not all codes apply to every single system, and some less common codes might not be listed.

13 – O2 (Oxygen Sensor) Circuit Open: Indicates an issue with the oxygen sensor circuit, potentially affecting fuel mixture and emissions.
14 – ECT/CTS (Engine Coolant Temperature) Sensor Circuit Low (High Temperature): Suggests the engine coolant temperature sensor is reading low voltage, which the computer interprets as high temperature.
15 – ECT/CTS (Engine Coolant Temperature) Sensor Circuit High (Low Temperature): Indicates the opposite of code 14 – the sensor is reading high voltage, interpreted as low temperature.
16 – VSS (Vehicle Speed Sensor) Buffer Fault: Points to a problem with the vehicle speed sensor buffer circuit, which can affect speedometer readings and transmission operation.
21 – TPS (Throttle Position Sensor) Circuit High (Signal Voltage High): Indicates the throttle position sensor is reporting a higher than expected voltage, even at idle.
22 – TPS (Throttle Position Sensor) Circuit Low (Signal Voltage Low): Indicates the throttle position sensor is reporting a lower than expected voltage.
24 – VSS (Vehicle Speed Sensor) Circuit Low: Suggests a low or no signal from the vehicle speed sensor.
28 – TR (Transmission Range) Pressure Switch Assembly: Indicates a problem within the transmission range pressure switch circuit.
32 – EGR (Exhaust Gas Recirculation) Valve Error: Signals an issue with the exhaust gas recirculation system, which is important for emissions control.
33 – MAP (Manifold Absolute Pressure) Sensor Circuit High (Low Vacuum): Indicates the MAP sensor is reading high voltage, suggesting low vacuum in the intake manifold.
34 – MAP (Manifold Absolute Pressure) Sensor Circuit Low (High Vacuum): Indicates the MAP sensor is reading low voltage, suggesting high vacuum in the intake manifold.
35 – IAC (Idle Air Control) Error: Points to a problem with the idle air control system, which regulates engine idle speed.
36 – Idle Speed Control Actuator Error (When Equipped): Similar to code 35, but specifically related to the idle speed control actuator if your vehicle is equipped with one.
42 – IC (Ignition Control) Error: Indicates an issue within the ignition control system.
43 – KS (Knock Sensor) Error: Signals a problem with the knock sensor circuit, which detects engine knocking or detonation.
44 – Lean Exhaust: Indicates the engine is running lean, meaning there is too much oxygen in the exhaust.
45 – Rich Exhaust: Indicates the engine is running rich, meaning there is too much fuel in the exhaust.
51 – PROM (MemCal, Chip) Error: Suggests a problem with the Programmable Read-Only Memory (PROM) chip, also known as the MemCal or chip, which contains the engine’s calibration data.
53 – System Voltage High (Supply to ECM/PCM): Indicates the system voltage supplied to the Engine Control Module (ECM) or Powertrain Control Module (PCM) is higher than expected.
54 – Fuel Pump Relay Circuit Low Voltage: Suggests a low voltage issue in the fuel pump relay circuit.
55 – ECM/PCM Error: Indicates a potential internal fault within the ECM or PCM itself.

Clearing OBD-I Codes on Your 91 GMC

After addressing the issue indicated by the trouble code, you’ll likely want to clear the code and turn off the “Service Engine Soon” light. OBD-I codes on your 1991 GMC can be cleared in a couple of ways:

Battery Disconnection: The simplest method is to disconnect the negative battery terminal for approximately 20 seconds. This will reset the computer and clear the stored codes.
Automatic Clearing: OBD-I systems are also designed to clear codes automatically if the problem that triggered the code does not reoccur after approximately 50 engine starts.

Conclusion: Empowering Your 91 GMC Diagnostics

Understanding and utilizing the OBD-I system in your 1991 GMC is a valuable skill for any owner. By following this guide, you can confidently retrieve trouble codes and use the provided code list as a starting point for diagnosing potential issues. Remember that while these codes offer valuable clues, further investigation and troubleshooting are often necessary to pinpoint the exact cause of the problem and ensure a lasting repair. This proactive approach to vehicle diagnostics can save you time and money, and keep your classic GMC running smoothly.