honda legend heat sensor code and s4 light

could you please help .... thanks

honda 91 replaced engine with an '88 model.
replaced original harness as engine wouldnt start.

the car runs perfectly but engine light remains on.

issue 1

computer shows code 6 - heat sensor
before replacing with the original sensor, i checked it by attaching to the plug - (the engine light went out), so i replaced the sensor.

the engine warning light and the s4 light went out.
ran the car for 20 minutes, no problems

when i restarted the car a third time an hour later, the engine light came back on, and the s4 light started flashing again



issue 2

the s4 light keeps flashing, the gears change up and down normally, i cant detect any problems

sigh