Transmission Issue K12 - OD/Off light blinking - no 4th Gear

Hello Everyone

I recently changed my Engine from CR12DEto CR14DE , keeping the same ECU and Gearbox , everything was fine for 2 weeks , car was running good but now it started to get issues with the transmission.

The OD/off light is blinking when startup and 4th gear not comming up. The car goes to the 1st gear, 2nd gear slips and 4th gear does not activate. the 3rd Gear can go up to 120 KMPH !!

I have changed the transmission Fluid to MATICJ as indicated on the gearbox Nozzle but same problem.

Any one can help to understand what is the issue exactly.
When you say "keeping the same ECU and gearbox" do you mean you kept the ECU from the 1.2 and are now using it with the 1.4?

It's unlikely that I am barking up the wrong tree but possible when I say the ECU is / could be your issue, the fuelling for the 1.4 would differ from the 1.2 so your lovely new 1.4 is only being fuelled as it it were the 1.2.

I think the gearboxes are the same, think being the operative word, at this point I am out of advice because I don't know how the fuelling would give you the what seems like mechanical gearbox issues.
Maybe the gearbox has some sort of inhibitor connected to the fuelling which controls the revs and therefore the ability to engage a gear that would be dangerous if it isn't getting the correct amount of fuel for safety reasons :unsure:👀
At first i thought that the car has some electrical issues , so i checked all the cables and fuse but the problem is still same. Yes , the ECU is from the 1.2 , I will try to find an ECU for 1.4 and have a try.
Thanks alot for your help