Computers and Control Systems: Description and Operation: Diagnostic Trouble Codes

Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of trouble codes as well as no trouble code problems. It is not necessary to perform all of the tests to diagnose an individual code.

Always begin by reading the diagnostic trouble codes using the DRB. This procedure begins in the DTC TEST - Checking the System for Diagnostic Trouble Codes. This will direct you to the specific test(s) that must be performed.

Hard Code
A diagnostic trouble code that comes back within one cycle of the ignition key is a hard" code. This means that the defect is there every time the powertrain control module checks that circuit or function. Procedures in this manual verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an "intermittent" test must be performed.

Codes that are for OBDII will not set with just the ignition key on. Comparing these to non-emission codes, they will seem like an intermittent. These codes require a set of parameters to be performed (The DRB III pre-test screens will help with this for MONITOR codes), this is called a "TRIP". All OBD II DTCs OBD II be set after one or in some cases two trip failures, and the MIL will be turned on. These codes require three successful, no failures, TRIPS to extinguish the MIL, followed by 40 warm-up cycles to erase the code.

Intermittent Code
A diagnostic trouble code that is not there every time the PCM checks the circuit is an "intermittent" code. Most intermittent codes are caused by wiring or connector problems Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them. The following checks may assist you in identifying a possible intermittent problem:

- Visually inspect related wire harness connectors. Look for broken, bent, Pushed out, or corroded terminals.
- Visually inspect the related harnesses. Look for chafed, pierced, or partially broken wire.
- Refer to any Hotline Newsletters or technical service bulletins that may apply.
- Use the DRB III data recorder or co-pilot.

Reset Counter
The reset counter counts the number of times the vehicle has been started since codes were last set, erased, or the battery was disconnected. The reset counter will count up to 255 start counts.

The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on the DRB as STARTS since set.

When there are no trouble codes stored in memory, the DRB will display "NO TROUBLE CODES FOUND" and the reset counter will show "STARTS since clear = XXX."

Diagnostic Trouble Codes (DRB)
For diagnostic trouble codes covered, refer to Trouble Code Descriptions.

Handling No Trouble Code Problems

Important Note: If the Powertrain Control Module has been changed and the correct VIN and mileage have not been programmed, a DTC will be set in the ABS and Air bag modules. In addition, if the vehicle is equipped with a Sentry Key Immobilizer Module (SKIM), Secret Key data must be updated to enable starting.

For ABS and Air Bag Systems:

1. Enter correct VIN and Mileage in PCM.
2. Erase codes in ABS and Air Bag modules.

For SKIM Theft Alarm:

1. Connect the DRB to the data link connector.
2. Go to Engine, Misc. and place the SKIM in secured access mode, by using the appropriate PIN code for this vehicle.
3. Select Update the Secret Key data, data will be transferred from the SKIM to the PCM (This is required to allow the vehicle to start with the new PCM).
4. If three attempts are made to enter secured access mode using the incorrect PIN, secured access mode will be locked out for one hour. To exit this lock out mode, leave the ignition key in the Run/Start position for one hour. Ensure all accessories are turned off. Also monitor the battery state and connect a battery charger if necessary.

After reading the System Description and Functional Operation, you should have a better understanding of the theory and operation of the on-board diagnostics, and how this relates to the diagnosis of a vehicle that may have a driveability-related symptom or complaint.

The "no code" system is broken down into three test methods:
- No Code Complete Test
- No Code Quick Individual Test
- No Code Quick Symptom Test

DRB Error Messages and Blank Screen
Under normal operation, the DRB will display one of only two error messages:

- User-Requested WARM Boot or User-Requested COLD Boot

If the DRB should display any other error message, record the entire display and call the scantool manufacturer for information and assistance. This is a sample of such an error message display:

ver: 2.14
date: 26 Jul93
file: key_itf.cc
date: Jul26 1993
line: 548
err: Ox1
User-Requested COLD Boot
Press MORE to switch between this display and the application screen.
Press F4 when done noting information.

DRB Does Not Power Up
If the LED's do not light or no sound is emitted at start up, check for loose cable connections or a bad cable. Check the vehicle battery voltage (data link connector cavity 16). A minimum of 11 volts is required to adequately power the DRB.

If all connections are proper between the DRB and the vehicle or other devices, and the vehicle battery is fully charged, an inoperative DRB may be the result of a faulty cable or vehicle wiring. For a blank screen, refer to the appropriate body diagnostics procedures.

DRB Scan Tool:

Display is Not Visible
Low temperature will effect the visibility of the display. Adjust the contrast to compensate for this condition.