Listings of the Diagnostic Trouble Codes (DTC's) for the various engine control system covered in this information are located in this section. Remember that a code only points to the faulty circuit NOT necessarily to a faulty component. Loose, damaged or corroded connections may contribute to a fault code on a circuit when the sensor or component is operating properly. Be sure that the components are faulty before replacing them, especially the expensive ones.
Keep in mind that most codes are only valid when the trouble is present. If the trouble is intermittent (often caused by loose or corroded terminals on circuit connections), it may lead to a dead end or "Intermittent code" ending in which the exact cause of the fault is still left undetermined. In these cases, check all related vacuum hoses and/or wiring for poor, worn or damaged connections. If bad connections are found, wiggle the connection in attempt to duplicate the fault condition so that troubleshooting may continue. Also, before replacing a computer control module, be ABSOLUTELY certain that the module is at fault. The price of module is usually quite significant and they are almost always NON-RETURNABLE components.
After making repairs, clear the trouble codes and operate the vehicle to see if it will reset, indicating further problems.
On 1982 models, a 5 terminal connector, located on the right side of the fuse panel, is used to activate the trouble code system in the ECM. This same connector is used at assembly and known as the Assembly Line Diagnostic Link (ALDL) or Assembly Line Communication Link (ALCL). A trouble code test terminal D is located in the 5 terminal connector and a ground terminal E , is located next to this terminal. With the ignition ON , car not running, jump the test terminal to ground.
On 1983-94 models, the ALDL connector or Data Link Connector (DLC), located under the dash, and sometimes covered with a plastic cover labeled DIAGNOSTIC CONNECTOR, can be activated by grounding test terminal B . The terminal is most easily grounded by connecting it to terminal A (internal ECM ground), the terminal to the right of terminal B on the top row of the ALDL connector. Once terminals A and B have been connected, the ignition switch must be moved to the ON position with the engine not running.
- Turn the ignition switch OFF .
- The Service Engine Soon or Check Engine light should be flashing. If it isn't, turn the ignition OFF and remove the jumper wire. Turn the ignition ON and confirm that light is now on. If it is not, replace the bulb and try again. If the bulb still will not light, or if it does not flash with the test terminal grounded, the system should be diagnosed by an experienced driveability technician. If the light is OK, proceed as follows.
- The code(s) stored in memory may be read either through counting the flashes of the dashboard warning lamp or through the use of a hand-held scan tool. If using the scan tool, connect it correctly to the ALDL.
- After the terminals are connected, turn the ignition switch to the ON position, but DO NOT start the engine. The dash warning lamp should begin to flash Code 12. The code will display as one flash, a pause and two flashes. Code 12 is not a fault code. It is used as a system acknowledgment or handshake code; its presence indicates that the ECM can communicate as requested. Code 12 is used to begin every diagnostic sequence. Some vehicles also use Code 12 after all diagnostic codes have been sent.
- After Code 12 has been transmitted 3 times, the fault codes, if any, will each be transmitted 3 times. The codes are stored and transmitted in numeric order from lowest to highest.
The order of codes in the memory does not indicate the order of occurrence.
- If there are no codes stored, but a driveability or emissions problem is evident, the system should be diagnosed by an experienced driveability technician.
- If one or more codes are stored, record them. Refer to the applicable Diagnostic Code chart in this section.
- Switch the ignition OFF when finished with code retrieval or scan tool readings.