How To Retrieve OBD I Trouble Codes (1986-1993 2.8L Chevy S10 Pickup, GMC S15 Pickup, GMC Sonoma)

The on-board diagnostic (OBD) system on pre-1996 Chevrolet S10 and GMC S15/Sonoma vehicles can display the trouble codes stored in the fuel injection computer by connecting two diagnostic link connector (DLC) terminals.

In this tutorial, I'll show you the location of the DLC and the specific terminals that need to be connected in order to retrieve the diagnostic trouble codes (DTCs) that are causing the check engine light (CEL) to come on.

You can find this tutorial in Spanish here: Cómo Leer Los Códigos De Falla (2.8L V6 GM) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 2.8L Chevrolet S10 Blazer: 1988, 1989.
• 2.8L Chevrolet S10 Pickup: 1988, 1989, 1990, 1991, 1992, 1993.
• 2.8L GMC S15 Jimmy: 1986, 1987, 1988, 1989.
• 2.8L GMC S15 Pickup: 1986, 1987. 1988, 1989, 1990.
• 2.8L GMC Sonoma: 1991, 1992, 1993.

Reading The On/Off Flashes Of The Check Engine Light

The diagnostic link connector can typically be found under the dashboard, usually on the right side of the steering wheel column.

In GM terminology, the Diagnostic Link Connector (DLC) is also known as the Assembly Line Data Link (ALDL) connector.

The connector is the same on all GM vehicles manufactured before the implementation of OBD II. The image at the beginning of this tutorial shows how the ALDL connector looks like.

You can use a paper-clip or a small piece of wire (with both ends stripped) to 'jumper' these two terminals together.

These are the steps to retrieve the trouble codes:

1. Make sure your battery is fully charged before starting (its customary to place a charger on the battery while retrieving trouble codes to keep the battery from discharging).
2. Turn the key to the ON position but don't start the vehicle.
3. Jumper terminals A and B, of the diagnostic link connector, together.
4. If no trouble codes are present:
   • The MIL will flash a code 12 (see illustration below) which consists of one flash, a short pause, then two flashes in rapid succession. Code 12 would look like this:
     • 'Flash', pause, 'flash', 'flash', long pause.
   • Code 12 will is repeated another 2 times (all codes are flashed 3 times before the next one).
5. If trouble codes are present:
   • The MIL will flash code 12 three times and then any other codes stored.
   • Each trouble code, in addition to code 12, is flashed 3 times.
   • A code 22 would look like this:
     • 'Flash', 'flash', pause, 'flash', 'flash', long pause.

Once you've confirmed and written down the trouble codes, remove the jumper wire from the diagnostic link connector (DLC) and turn the key to the Off position.

GM OBD I Diagnostic Trouble Code Definitions

Chevy S10/GMC S15 OBD I Trouble Code Definitions
Code	Description
12	System Check
13	Oxygen Sensor Circuit
14	Engine Coolant Temperature (ECT) High Temperature Indicated
15	Engine Coolant Temperature (ECT) Sensor Signal Voltage High
16	Transmission Speed Error
21	TPS Signal Voltage High
22	TPS Signal Voltage Low
23	MAT (Manifold Air Temperature) Signal Voltage High (2.5L Only)
24	Vehicle Speed Sensor (VSS) Signal Voltage High
25	MAT (Manifold Air Temperature) Signal Voltage Low (2.5L Only)
26	Quad Driver 1 Fault (4.3L M/T only)
27	Quad Driver 2 Fault (4.3L M/T only)
28	Transmission Pressure Switch Manifold Range Circuit Problem
32	EGR System Failure
33	MAP Sensor Signal Voltage High
34	MAP Sensor Signal Voltage Low
35	Idle Speed Error
37	Brake Switch Stuck On (4L60-E A/T only)
39	Knock Sensor Circuit Shorted (4.3L M/T Only)
42	Electronic Spark Timing (EST) Fault
43	Electronic Spark Control (ESC) Fault
44	Lean Exhaust
45	Rich Exhaust
51	PROM/MEM-CAL Fault
51	System Voltage High (Charging System Problem)
54	Fuel Pump Circuit
54	Faulty ECM/PCM/VCM

More 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials

You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:

• GM 2.8L Index Of Articles.

Here's a sample of the articles, you'll find in the Index of Articles:

• How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test Engine Compression (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
• How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).