Pcm DTC EO23: Est Circuit Problem: Description

For complete description of how ignition spark is controlled by the PCM and the HEI module, refer to THEORY & OPERATION article. Code EO23 will set under either of the two following conditions:

1. Reference pulses are being received by the PCM and the bypass line is low, and the PCM detects pulses on the EST line. This could be caused by the bypass line shorted to voltage at the distributor.
2. Reference pulses are being received by the PCM and the bypass line is high, and the PCM does NOT detect a signal on the EST line. This could be caused by an open or a shorted to ground EST line or bypass line open or shorted to ground.