GM Buick/Oldsmobile/Pontiac/Full-Size 1975-1990 Repair Guide



See Figures 1, 2, 3 and 4

The following explains how to activate and erase the trouble codes using the Check Engine light in the instrument cluster. This is not a full fledged C-4 or CCC system troubleshooting and isolation procedure.

Before suspecting the C-4 or CCC system, or any of its components as being faulty, check the ignition system (distributor, timing, spark plugs and wires). Check the engine compression, the air cleaner and any of the emission control components that are not controlled by the ECM. Also check the intake manifold for any leaks. Check the carburetor mounting bolts for tightness. Inspect all vacuum hoses for correct routing, pinching by a harness tie or any object in the engine compartment, cuts, cracks, or loose connections. Make sure to follow them under the air cleaner, generator, or other object to make sure they are in good condition and not pinched along their entire length. Inspect all wiring in a similar manner, checking for chafing of the insulation, burned spots, pinching (which could ground a wire), contact with any sharp edge, or routing too near any hot portion of the engine. Also check that all connections are clean and tight. This visual inspection is extremely important as many operating problems can be cleared up only by repair of bad wiring or vacuum hoses.

Trouble codes tell you there is a problem somewhere in a certain circuit. Because of this, the exact diagnosis of a problem is a very complex procedure involving expensive equipment and should be left to a qualified technician. Once you retrieve a code, you should never just simply replace a sensor in a circuit, as most trouble codes are incurred due to poor connections or bad wiring. Instead, perform a visual inspection of the connectors and wiring of that circuit. If the problem is not found, leave further diagnosis to an experienced technician.

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Fig. Fig. 1: C-4 trouble code identification chart

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Fig. Fig. 2: CCC trouble code identification chart (Carbureted)

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Fig. Fig. 3: CCC trouble code identification chart (Fuel Injected)

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Fig. Fig. 4: Diesel trouble code identification chart


See Figures 5 and 6

As a bulb and system check, the Check Engine light will come on when the ignition switch is turned to the ON position but the engine is not started.

The Check Engine light will also produce the trouble code/codes by a series of flashes which translate as follows: When the diagnostic test lead (C-4) or terminal (CCC) under the instrument panel is grounded, with the ignition in the ON position and the engine not running, the Check Engine light will flash once, pause, and then flash twice in rapid succession. This is a Code 12, which indicates that the diagnostic system is working. After a long pause, the Code 12 will repeat itself two more times. This whole cycle will then repeat itself with any other codes stored in memory until the engine is started or the ignition switch is turned OFF. If it only flashes a Code 12, there are no stored trouble codes.

When the engine is started, the Check Engine light will remain on for a few seconds and then turn off. If the Check Engine light remains on, the self-diagnostic system has detected a problem. If the test lead (C-4) or test terminal (CCC) is then grounded (stop the engine but leave the ignition ON), the trouble code will flash (3) three times. If more than one problem is found to be in existence, each trouble code will flash (3) three times and then change to the next one. Trouble codes will flash in numerical order (lowest code number to highest). The trouble code series will repeat themselves for as long as the test leads or terminal remains grounded.

In the case of an intermittent fault in the system, the Check Engine light will go out when the fault goes away, but the trouble code will remain in the memory of the ECM. Therefore, if a trouble code can be obtained even though the Check Engine light is not on, it must still be evaluated. It must be determined if the fault is intermittent or if the engine must be operating under certain conditions (acceleration, deceleration, etc.) before the Check Engine light will come on. In some cases, certain trouble codes will not be recorded in the ECM until the engine has been operated at part throttle for at least 5-18 minutes.

On the C-4 system, the ECM erases all trouble codes every time that the ignition is turned off. In the case of intermittent faults, a long term memory is desirable. This can be produced by connecting the orange connector/lead from terminal S of the ECM directly to the battery (or to a "hot" fuse panel terminal). This terminal must always be disconnected immediately after diagnosis as it puts an unnecessary drain on the battery.

On the C-4 system, activate the trouble code by grounding the trouble code test lead. Use the illustrations to help you locate the test lead under the instrument panel (usually a white and black wire with a green connector). Run a jumper wire from the lead to a suitable ground.

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Fig. Fig. 5: C-4 diagnostic lead location

On the CCC system, locate the test terminal under the instrument panel (see illustration). Use a jumper wire and ground only the leads. Jumper B to A on all models where letters run from F to A going from right to left.

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Fig. Fig. 6: CCC diagnostic lead location


The ignition must ALWAYS be in the OFF position when connecting or disconnecting power from the ECM. The module could be destroyed if this is not followed.

Codes can be cleared by using any of the three following procedures:

  1. Removing the ECM fuse from the fuse panel for 30 seconds.
  3. Disconnecting the ECM pigtail near the battery.
  5. The codes can also be cleared by disconnecting the negative battery cable, however, it will clear other memory systems such as the clock and radio as well.

If the codes reappear after clearing, it indicates the trouble area has not been repaired.