Engine light blinking key on position.
When a key is just on the ON position (engine not run yet) the sign CHECK ENGINE SOON already blinking very fast. When I start the engine, light continues blinking. I don't drive the car. Engine running well., idle about 750-800 rpm, showing good power when I push the gas pedal. When I pulled error codes which shows first 3 x code 12, so is normal. Only one code 51(book shows PROM Error Checked pins for bending and good sitting. Don't know if I should replaced PROM. Otherwise car running ok on the ground
The car has a new heavy ground cable installed from battery to the engine block. Battery has very good holding power at the starting. I understand when the car is running it might be many possibilities of the blinking this light but the just key On position don't understand. Thx for help.

 

If the SES lamp flashes rapidly it could also indicate the ECM in Field Service Mode. That is normally only invoked when the engine is running, however.

It could also indicate a PROM checksum error, and the ECM is using the backup parameters to run the engine. In addition to checking the PROM connections (inside the CALPAK), check the complete CALPAK for reliable connections.