Pcm Code EO23, Est Circuit Problem (1 Of 2): Action:

PCM turns on SERVICE ENGINE SOON light and will not enable spark timing control. The engine will start and run on base timing.

1. When circuit No. 423 is shorted to ground, engine will have a long crank and hard start. When PCM detects grounded circuit, by-pass line will be shifted low.
2. With only distributor reference (distributor reference ground jumpered), engine will run on back-up spark control. This checks proper ground connections between PCM and engine, and PCM and HEI.
3. This step ensures PCM provides EST output signal over circuit No. 423.
4. This checks if HEI module can ground EST signal with an open by-pass circuit.

Fig 1: PCM Code EO23, Schematic, EST Circuit Problem

Fig 2: Flow Chart 1 Of 2, PCM Code EO23, EST Circuit Problem