6.5 Diesel OBD I Code Reading Procedures & DTC List

Applicable Models:

1993 - 1995 Chevrolet & GMC C/K series trucks, Suburban, etc

Applicable Engine(s):

6.5L GM diesel

OBD I Diagnostic System Overview

The 6.5 diesel adopted the 1st generation on-board diagnostic protocol for the 1993 model year. The OBD I system was carried over through the 1995 model year and replaced by the more sophisticated OBD II protocol in 1996. OBD I differs considerably from later OBD II systems. The OBD I protocol is simpler, offers significantly less functionality, and is not standardized across vehicle makes. The OBD II protocol, which offers sophisticated system monitoring and special diagnostic test functions, remains the industry standard today.

The purpose of the OBD I diagnostic system is to warn drivers of potential vehicle faults (primarily engine and transmission related) and provide a method of storing and retrieving said faults. A stored fault is referred to as a DTC - diagnostic trouble code - and for OBD I systems it is a simple numeric value that corresponds to a single fault. Certain engine problems therefore produce multiple DTCs, which are stored and retrieved individually. These codes generally represent pieces of a puzzle that a technician can use, combined with other diagnostic procedures, to pinpoint problems.

For 6.5 diesel vehicles, DTCs stored by the OBD I system can be retrieved with either a special diagnostic tool (code reader) or simply by initiating the on-board diagnostic sequence. Note that anytime the "Service Engine Soon" light located to the left of the tachometer in the gauge cluster is illuminated, at least one DTC is actively stored. Initiating the on-board diagnostics sequence on a 6.5 diesel is simple and requires no code reader.

OBD-I Connector Location for 6.5 GM Diesel Vehicles

The OBD I system is accessed through a 12 pin ALDL connector mounted just beneath the dash to the left of the steering column. If the connector is not affixed to the bottom of the dashboard, check behind the dash where it should be; the mounting tabs may have broken off and a previous owner or technician could have stashed it with the bundle of wires behind the lower section of the dashboard.

OBD I connector location on 6.5 diesel — Figure 1 - OBD I connector location

OBD-I Connector Pinout

Connecting terminals "A" and "B" on the OBD I connector initiates the diagnostic sequence. Often referred to as "jumping", this can be done with any conductive material, but a small diameter copper wire is preferred.

How to Read DTCs on a 6.5 Diesel

DTCs are identified by monitoring the "Service Engine Soon" (SES) light after initiating the diagnostic sequence. Once the diagnostic mode has been established, codes will be revealed using a system similar to Morse Code, whereby you will count the number of times the SES light flashes and this corresponds to one numerical value of the code being identified. Short pauses between illuminations indicates the system is still distinguishing a specific number while a long pause indicates that that number sequence has completed. For the following example, let a single dash ("-") indicate a short pause, three sequential dashes ("---") indicate a long pause, and "SES" indicate that the Service Engine Soon light is briefly illuminated:

SES---SES-SES would indicate the number "1" followed by the number "2", thus the DTC equals 12.
SES-SES---SES-SES-SES would indicate the numbers "2" and "3", thus the DTC equals 23.
SES-SES-SES---SES-SES-SES-SES-SES would indicate the numbers "3" and "5", thus the DTC equals 35.

6.5 diesel service engine soon light (SES) — Figure 3 - 6.5 diesel "Service Engine Soon" light

All DTCs are indicated in numerical order and repeat themselves three times before cycling to the next stored DTC. The DTC 12, which is simply a check code verifying that the system is functioning properly, will display three times before any stored DTCs are displayed. If there are no stored DTCs, DTC 12 will continue to display perpetually. To enter the diagnostic mode:

With the ignition switch in the "OFF" position, connect the "A" and "B" terminals from figure 2 above using a short length of wire (16 AWG solid core wire stripped at both ends works well, but there are obviously many methods of jumping the two terminals).
Switch the ignition switch to the "RUN" position, which will initiate the on-board diagnostic code retrieval sequence.
Monitor the "Service Engine Soon" light, which will indicate the DTC 12 three times, followed by any additional codes.
Turn the key to the "OFF" position and remove the wire jumper between terminals "A" and "B" when you are done reading codes.

The sequence will cycle perpetually so long as the key is the "RUN" position, the engine is off, and there is electrical contact between the A and B terminals of the OBD I connector. If you missed a number or need to verify a DTC, simply let the sequence start over again (it will always start with DTC 12 three times). If the SES light does not illuminate, verify that the A and B terminals are connected together. Other possible problems include a burnt out SES light bulb or faulty wiring to/from the OBD I connector.

To recap:

Keep counting the number of SES light illuminations so long as the pause between illuminations is short. When you come to a long pause, that number is complete and the next one is about to begin.
Each trouble code will be displayed a total of three times before moving on to subsequent codes.
DTC 12 will be displayed three times at the beginning of each sequence, and the process will loop perpetually until the key is turned off.
Codes range from 12 (the smallest) to 99 (the largest), but there is no zero and a few of the numbers between 12 and 99 are skipped. Note that there is no single digit codes; all codes have two digits.
Codes will always be displayed in numerical order.

6.5 Diesel OBD-I Code List

12 - Diagnostic system functioning properly (check code)
13 - Engine shutoff solenoid circuit fault
14 - Engine coolant temp sensor circuit low (high temp)
15 - Engine coolant temp sensor circuit high (low temp)
16 - Vehicle speed sensor buffer fault
17 - High resolution circuit fault
18 - High resolution circuit fault
19 - Crankshaft position reference fault
21 - Accelerator pedal position 1 circuit high
22 - Accelerator pedal position 1 circuit low
23 - Accelerator pedal position 1 circuit out-of-range
24 - Vehicle speed sensor circuit low
25 - Accelerator pedal position 2 circuit high
26 - Accelerator pedal position 2 circuit low
27 - Accelerator pedal position circuit out-of-range
28 - Transmission range pressure switch circuit fault
29 - Glow plug relay fault
31 - EGR control pressure/barometric sensor circuit low
32 - EGR circuit fault
33 - EGR control pressure/barometric sensor circuit high
34 - Injection timing stepper motor fault
35 - Injection pulse width error, short duration (pulse width shorter than desired)
36 - Injection pulse width error, long duration (pulse width longer than desired)
37 - Torque converter clutch (TCC) brake switch fault, stuck "ON"
38 - Torque converter clutch (TCC) brake switch fault, stuck "OFF"
39 - Torque converter clutch (TCC) stuck "OFF"
41 - Brake switch circuit fault
42 - Fuel temperature circuit low (temp high)
43 - Fuel temperature circuit high (temp low)
44 - EGR pulse width error
45 - EGR vent error
46 - Malfunction indicator lamp (MIL) circuit fault
47 - Intake air temperature sensor circuit low (temp high)
48 - Intake air temperature sensor circuit high (temp low)
49 - Service throttle soon lamp circuit fault
51 - P.R.O.M. error
52 - System voltage high, long
53 - System voltage high
54 - PCM fuel circuit error
56 - Injection pump calibration resistor error
57 - PCM 5 volt shorted
58 - Transmission fluid temperature circuit low (high temp)
59 - Transmission fluid temperature circuit high (low temp)
61 - Turbocharger boost sensor circuit high
62 - Turbocharger boost sensor circuit low
63 - Accelerator pedal position 3 circuit high
64 - Accelerator pedal position 3 circuit low
65 - Accelerator pedal position 3 circuit out-of-range
68 - Transmission slip detected
69 - Torque converter clutch (TCC) stuck "ON"
71 - Cruise control set/coast switch fault
72 - Vehicle speed sensor circuit loss
73 - Pressure control solenoid circuit error
74 - Transmission input speed sensor circuit fault
75 - System voltage low
76 - Cruise control resume/accelerate switch fault
78 - Turbocharger wastegate solenoid fault
79 - Transmission fluid over temperature
81 - Transmission 2-3 shift solenoid circuit fault
82 - Transmission 1-2 shift solenoid circuit fault
83 - Torque converter clutch fault
84 - Accelerator pedal position circuit fault
85 - Transmission gear error
86 - Transmission gear ratio error
87 - Transmission gear ratio error
88 - Top dead center offset error
91 - Cylinder balance fault, cylinder 8
92 - Cylinder balance fault, cylinder 7
93 - Cylinder balance fault, cylinder 6
94 - Cylinder balance fault, cylinder 5
95 - Cylinder balance fault, cylinder 4
96 - Cylinder balance fault, cylinder 3
97 - Cylinder balance fault, cylinder 2
98 - Cylinder balance fault, cylinder 1
99 - Accelerator pedal position sensor reference voltage error (5 volt ref)

Summary

While not nearly as sophisticated as modern diagnostic systems, the OBD-I protocol is designed to help identify the cause of potential powertrain problems.
Reading codes on a 6.5 diesel is accomplished by counting the number of times the "Service Engine Soon" light flashes in sequence.
OBD codes are merely a starting point in the process of troubleshooting powertrain related problems.