Optispark Troubleshooting

optispark optispark failure optispark problem optispark troubleshooting

  • How do you know the Optispark is failing?

    Let’s envision you are out in city traffic. As heat builds under the hood you first notice a minor misfire, then stalling occurs. You are most likely able to start the engine back up, but as time goes by the wait time increases before the engine will start. Eventually the engine will run when it is cool but as soon as heat builds it dies and refuses to start no matter what the engine temperature is.

    Before you rush to replace a component, be advised that the 1992-1993 ECM (Engine Control Module) is well noted for symptoms that resemble a failing Optispark. The difference is that ECM failures are usually not heat dependent and occur more randomly. In many instances you can go out and rap the ECM with a closed fist and the engine can be started, or if running at the time it will stop running.

    Retrieving Codes

    The first step is to check for any DTCs (Diagnostic Trouble Codes). To find engine codes you can either use a code reader, scanner, or in the case of the Corvette, a short section of wire via the ALDL (Assembly Line Diagnostic Link). A paperclip works fine in a pinch. All 1992-1993 GM vehicles use the ALDL to allow you to connect and retrieve codes. To display codes place the jumper wire in ALDL position B and A, Position A is at the top right corner and position B is to the left of A in the same row. Now turn the ignition to the ON position. The SES light will flash code 12 first to indicate that you have successfully entered the diagnostic mode. A code 12 is shown with one flash, a pause, and then two flashes. Code 12 will be shown three times before any other codes are displayed.

    OBDI began in 1994 and by 1996 OBDII arrived. Vehicles equipped with OBDII require a scanner or code reader to retrieve codes in all vehicles after 1993. OBDI code readers will not work with OBDII systems so be sure to purchase the correct code reader or scanner for your application. In the case of the 1992-1996 Corvette, engine codes can be accessed with a short piece of wire. The codes will be displayed via the speedometer read-out area and controlled at the information center. The info below will walk you through the Corvette code display procedure.

    1990-1994 Corvette Code Retrieval Procedures

    All ECM, CCM and ASR/EBTCM codes can be accessed by placing a jumper wire in ALDL position G and A. Position A is at the top right corner and position G is at the lower left of the ALDL. The ALDL scanner link is located above your right knee when you are seated in the car and about four inches from the side of the console. The CCM will display all module codes in automatic sequence in the speedometer read-out area starting with module 1 CCM (Central Control Module) codes then module 4 ECM (Engine Control Module) codes, finishing with module 9 ABS/ASR EBTCM (Anti-lock Brake System) / (Acceleration Slip Regulation) (Electronic Brake Traction Control Module) codes. The particular module number (1, 4, or 9) will display in the lower left directly below the speedometer read-out area. The codes themselves will be displayed in place of the speedo read-out with a prefix beginning with an “H” for “History” or “C” for “Current,” then the code. Once all module codes are displayed, “—“ is displayed, ending the automatic sequence. You can turn the ignition off and back on to start the automatic code display again if you miss any codes. By the way, for those of you with LT5 engines, the CCM will not display the ECM codes- a scanner must be used. This may be too obvious, but the “C” is for current codes meaning the problem is occurring in real time while “H” is for history. It is always best to record all the codes, clear them, then run the engine and see if any codes reoccur. Clearing codes can be done after the automatic sequence of codes has occurred using the following instructions.

    Clearing Codes

    To clear the ECM codes (also known as Service Engine Soon codes) after the automatic sequence has been completed wait until the speedometer display is blank and the trip monitor display reads 1.0. Press the TRIP RESET button until 4.0 is displayed on the trip monitor, then press the TRIP/ODO button until 4.7 is displayed. Finally, press the ENG/MET button until “—“ is displayed in the speedometer display.

    Codes can also be cleared by removing the negative battery cable for at least 30 seconds.


    To clear CCM codes, press the TRIP RESET button until 1.0 is displayed in the module number area. Then press the TRIP/ODO button until 1.7 is displayed in the module number area. Once there, press and hold the ENG/MET button until “—“ is displayed. This will clear any CCM codes. After the “—“ is displayed turn the key off and wait ten seconds, then turn the key back on and check for any CCM codes.


    Press the TRIP RESET button until 4.0 is displayed in the module number area. Then press the TRIP/ODO button until 4.7 is displayed in the module number area once there press and hold the ENG/MET button until “—“ is displayed. This will clear any ECM codes after the “—“ is displayed turn the key off and wait ten seconds turn the key back on and check for any ECM codes.


    Press the TRIP RESET button until 9.0 is displayed in the module number area. Then press the TRIP/ODO button until 9.7 is displayed in the module number area. Once there, press and hold the ENG/MET button until “—“ is displayed. This will clear any ABS/ASR codes. After the “—“ is displayed turn the key off and wait ten seconds, then turn the key back on and check for any ABS/ASR codes.

    Confirm Codes

    After clearing the codes go back to the automatic code display sequence to see if any current codes are present. A hard fault is occurring presently if any codes reappear.

    1995-1996 Corvette Code Retrieval Procedures

    Retrieving and clearing the codes requires a jumper wire to ground terminal 12 to terminal 4 of the data link connector located on the lower driver’s side panel just above your right knee when sitting in the seat. The DLC (Data Link Connector) has 16 terminals. Number 12 is on the bottom row, four spaces over from the left. There are numeric codes on the DLC, but they are hard to see. The ground terminal position 4 in the DLC is directly above position 12. A short jumper wire can be used to connect position 4 and 12. Place the jumper wire in position and turn the ignition on with the engine not running.

    When the ignition is turned on all module codes will be displayed in automatic sequence starting with the CCM (Central Control Module), PCM/ECM (Powertrain Control Module/Engine Control Module), ABS/TCS EBCM (Antilock Brake System/Traction Control System) (Electronic Brake Control Module) in the speedometer LCD screen. The CCM is module 1 and any codes will be displayed in the speedometer area. The first codes will be current codes and then history codes. All the modules will follow the same criteria and display any codes (current and history). When the automatic sequence is over the ignition key can be turned to the off position and back to ignition on and the automatic sequence will begin again so do not worry if you miss a code.

    To clear the PCM codes (Service Engine Soon) after the automatic sequence has been completed the speedometer display will be blank and the trip monitor display will read 1.0. Press the TRIP RESET button until 4.0 is displayed on the trip monitor, then press the TRIP/ODO button until 4.7 is displayed, then finally press the ENG/MET until “—“ is displayed in the speedometer display.


    Before condemning the Optispark for a no-start condition there are a few things that should always be checked.

    • Make sure you have fuel pressure and the injectors are not shorted on 1992-1993 GM vehicles.
    • You should be able to hear the fuel pump run for a few seconds after the ignition switch is turned to the run position. This usually requires a helper to listen or operate the ignition, remove the fuel cap and listen carefully for fuel pump operation.
    • In some cases the pump will run but not pressurize the system. In that case you need to use a fuel pressure tester. If you are stranded somewhere, lightly depress the fuel pressure test port shrader valve located in the fuel rail. Fuel should spray out; if it dribbles out the pump is not working. Be careful doing the manual check without the fuel pressure gauge because the fuel can come out and spray onto heated surfaces and cause a fire.
    • Fuel injectors with shorted windings will shut down the engine and it can happen when you least expect it. You can be driving down the road and fill the fuel tank and never start the engine again until the injector/injectors are replaced.
      • An ordinary 12 volt test light can be used to check for a shorted injector.
      • Place the test light’s ground clip on a known-good ground on the engine, then probe the injector connector (the injector connectors are to remain connected to the injectors during this test). To get an idea of how bright the light should be, touch the test light probe directly to the battery; this will also assure that the light is working. The fuel injector connectors’ two wires control the fuel injectors’ length of “on time.” One wire is for ignition voltage and the other is coming from the ECM grounding the wire to open the injector. The test light cannot hurt the ECM if you touch the wire that does not light the test light. Probing one side of the fuel injector connector terminals will cause the test light to light. Crank the engine over. The light should be bright; if not, one or more injectors have shorted out. To be sure, remove all the injector connectors and try the test light again; the light should be bright at all of the connectors.
      • If you are stranded you can connect one injector at a time and try the test light as each one is connected. When you connect the shorted injector the test light will dim, an indication that you have found the bad injector. Usually only one or two injectors fail at a time, and leaving the failed injectors disconnected will allow the engine to run. The engine will lose power but it will not hurt the engine because no fuel is being introduced into the cylinders.

    If you find an ECM code 16 and the engine will not run the Optispark has failed and will need to be replaced.

    If you find a code 36 with the engine is running poorly, the most probable cause is a failed Optispark. Code 36 occurs when the ECM does not see a high resolution pulse from the Optispark. The 1992-1993 ECM uses the high resolution pulse in conjunction with the low resolution pulse to control fuel injection and ignition control. The high resolution pulse is used to control the fuel injectors sequentially, which did not occur until 1994 on all GM LT engines. The 1994-1995 engine operation will be greatly affected by the loss of the high resolution pulse, effectively putting it into “limp-home” mode (poor performance and lack of power). By 1996, GM added a crankshaft position sensor on the LT engines to recognize the sequential fuel injectors’ pulse requirements and they would display a different code if there was a resolution pulse error: code P0323 low/high resolution circuit intermittent, code P0335 crankshaft position sensor circuit failure, or P01371 low resolution circuit failure. If you find a code P0323 or P01371, the Optispark has most likely failed.

    Wiring, Connectors, and Terminals  

    Keep in mind that an LT equipped GM vehicle is at least 17 years old and all of the underhood wiring and connectors have been subjected to many years of high heat. Also, every time a connector is disconnected the terminals are subjected to possible damage; try to avoid multiple disconnections and connections. We have often found terminals that are opened up and causing a poor connection that is very difficult to diagnose. We always inspect and check each terminal for proper retention during our diagnosis. In the case of intermittent no starts or codes that cannot be explained (engine runs fine but codes appear) we check each and every terminal for integrity. We can provide you with the proper terminals to check for a possible terminal retention problem. Please use the contact form on our website to request them.

    High Voltage Concerns

    The Optispark distributor was designed to handle spark voltage in the 40,000-45,000 range at peak load. As spark plugs wear, their gap increases, requiring more voltage to bridge the gap. The distributor cap and rotor are required to handle the additional load to bridge the gap of eight worn out spark plugs. Spark plug wires also wear as they age gaining more resistance and requiring higher voltage to ignite the spark plugs. The ignition coil itself works tirelessly supplying voltage at idle and anywhere in between. After 100,000 miles ignition coils become tired. Ignition coils have an even tougher existence when they are supplying voltage to worn components.  To assure your new Optispark will live a long life, spark plugs, spark plug wires, and possibly even the ignition coil should be replaced; absolutely if the engine has over 100,000 miles. Always replace the spark plug wires using new retainers if yours are broken. Correct plug wire routing is paramount for maximum engine performance and plug wire life. Plug wires lying on hot exhaust manifolds will shorten their life and cause high voltage loads on the entire ignition system.

    The use of aftermarket high voltage products will shorten the life of the distributor cap and rotor. For high performance applications we recommend the use of high output ignition coils. Be aware that the distributor cap and rotor life will be shortened and should be checked dependent on how you use your vehicle. For example, if you use a high output coil for an everyday driver and rarely operate at high RPM you should be able to go 75,000-100,000 miles without concerns. If you race or subject your engine to high RPM frequently you may need to replace the cap and rotor annually.

    The installation of an aftermarket ignition box that produces multiple sparks also raises the available spark voltage. Unlike an aftermarket ignition coil which supplies additional spark voltage at wide open throttle, the multiple spark unit supplies additional coil voltage at all times. This means that at idle the coil’s input voltage could be boosted to 18.0 volts whereas the normal operating voltage would be 14.0-14.6 volts. Even a stock ignition coil would raise the coil output higher from idle to wide open throttle shortening the life of the distributor cap and rotor. However, our brass contact cap and rotor will withstand the additional voltage better than the factory components by a great margin. We offer ventilated brass contact distributor caps and rotors available when replacement is required.


    We hope that you have found this Optispark troubleshooting guide informative and helpful. We offer affordable consultations should you require further assistance in troubleshooting a possible Optispark failure or have any other technical questions.

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