Code 26, Diagnostic Circuit Check

The Diagnostic Circuit Check determines:

• If SERVICE ENGINE SOON light works
• If ECM is operating and can recognize a fault
• If any codes are stored
  1. Check operation of SERVICE ENGINE SOON light. Turn ignition on with engine off. SERVICE ENGINE SOON light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to SERVICE ENGINE SOON LIGHT INOPERATIVE. If light flashes, go to step 3.
  2. Grounding ALDL "test" terminal "B" at this time should cause SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. If light foes from bright to dim, this is not considered a code. If light dims or remains on, and does not flash Code, go to SERVICE ENGINE SOON LIGHT WON'T GO OUT OR FLASH CODE 12.
  3. If light begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between ALDL terminal "B" and ECM terminal No. 5. If circuit is okay, replace ECM.