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"Non-Scan" Diagnostic Circuit Check

The diagnostic circuit check is an organized approach for identifying a problem caused by the fuel injection system. Driver complaints fall into 3 categories: steady "SERVICE ENGINE SOON" light, driveability problems, and "engine cranks, but will not run". Understanding chart and using it correctly will reduce diagnostic time and prevent unnecessary replacement of parts.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).
  1. A steady "SERVICE ENGINE SOON" light with ignition on and engine not running confirms battery and ignition voltage to Electronic Control Module (ECM).
  2. Ground diagnostic test terminal by connecting a jumper wire between terminals "A" and "B" in assembly line communication link (ALCL) connector, located below instrument panel. The ECM will cause "SERVICE ENGINE SOON" light to flash Code 12, indicating that ECM diagnostics are working. Code 12 will flash 3 times, followed by other trouble codes stored in memory. Each additional code will flash 3 times, starting with lowest code, and then start over again with Code 12. If there are no other codes, Code 12 will flash until diagnostic test terminal jumper is disconnected or engine is started.
  3. Record all stored codes, except Code 12. If the problem is "engine cranks but will not run", proceed to CHART A3.
  4. If no additional codes were recorded, see TROUBLE SHOOTING procedures in CEC TESTS W/O CODES article in this section for driveability symptoms and recommended service procedures. With engine running and diagnostic terminal grounded, the ECM will respond to the oxygen sensor signal and use the "SERVICE ENGINE SOON" light to display the following information:

    A)  Closed loop confirms that oxygen sensor signal is being used by the ECM to control fuel delivery and that system is working properly. Signal voltage will vary from below .35 to above .55 volts.

    B)  Open loop indicates that oxygen sensor signal is not useable to ECM. Signal voltage will be constant and between 35 and .55 volts. System will flash "open loop" for 30 seconds to 2 minutes after engine starts or until sensor reaches normal operating temperature. If system fails to go into closed loop, see Code 13 test.

    C)  "SERVICE ENGINE SOON" light off indicates that exhaust is lean. Oxygen sensor will be less than .35 volts and steady. See Code 44 test procedures.

    D)  "SERVICE ENGINE SOON" light on steady indicates that exhaust is rich. Oxygen sensor signal will be above .55 volts and steady. See Code 45 test procedures.

  5. Road test of the system in the field service mode should be done only at steady road speeds. The following conditions may be observed and should be considered normal: light on too long under acceleration, light off too long under deceleration or light on too long with idle below 1200 RPM.
  6. To clear codes, turn ignition off and disconnect battery pigtail for 10 seconds
Fig 1: "Non-Scan" Diagnostic Circuit Check
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