Limp mode

[Eddy Lau]

Hi , am new to the ae86, and my Trueno ran perfectly until recently.
It got itself into limp mode I think. I can’t rev over 3000rpm and the check engine light come on as if I hit the limited. If I let off the throttle, the revs dropped and check engine light goes off too. It ran smoothly and sweet below 3000rpm everything is as it should no problem in starting.
I did a diagnostic test and the codes 3 , 5 and 9 came up.
3 is the Ignition signal to ecu
5 is the oxygen sensor
9 is the vehicle speed signal to ecu which does not trigger a CEL
My question is does the fault code 3 or/and 5 trigger the limp mode or there might be other reason?
A new oxygen sensor is on order but not sure what parts to replace regarding the ignition signal to ecu.
Any help is appreciated.