How can you tell if a new gearbox is needed other than a Ecu which has been replaced

What make , model an year vehicle ? What codes were pulled , if any ?

didnt specify vehicle --generic answer--until repair is made the vehicle will show engine light and have codes --if gearbox is problem for limp mode /until repairs are made or gearbox replaced you will be in limp mode --and as to sensor find out what one it was --start problem may show back up if sensor was problem --and GET a 3rd opinion you have 2 mechanics with different opinions on your problem and i guess they are reading your trouble codes differently

The linkage needs adjusting between the shifter and the transmission. There's usually a visible adjusting point under the hood on a front wheel drive vehicle, by the front of the transmission.

you might need a fuel pump relay under dash 10 min fix/and your d4 flashing light bad transmission.

ooooooh is that a hard question.
go to the cars web side, go to parts , look up the ECU
for both,
or go to the dealer and ask.
in newer cars, the answer is always yes and no.
no , because you can not flash your options into the replacement.
only factory tools can do that.

if the 2 cars are identical in every way, ID BET NOT.
then maybe.
the ECU parts lists shows the base ECU p/n and tells you that fact. on line they love to leave off p/n. (sux)
so you must walk in to the dealer, and look on the screen there.
or ask.
them Can my base 05 ECU work in an 06 ECU.
in some cases, (calif) its flat illegal. to down grade any ECU.
and the smog stations can tell you did that. scanning it.

on old cars this was easier, 6 variants.
but now , huge numbers of options on cars...
in some cases the new ECU must be married to the Dash odometer puter. of the odometer is born dead,
and is a felony in some states to monkey the odometer.
this post gives just the tip of the ice berg.

hi this problem is caused by a geatbox fault , something is causing the fault code to bring on your warning lights , in this mode the gearbox locks up and the tourqe converter takes up the drive , when you drive of it feels like you only have 3 gear , change the oil and filter in the box and make sure you have the correct level of oil in the box , very important , td5 auto 192k

I'm having a similar problem with my '05 A4 Multitronic with 75,000 miles on clock. PRNDS lights are all blocked out but not flashing (which I believe is worse). Car is driving ok after I had the EGR valve replaced by dealer at cost of £454. Dealer quoted me a further £1,077 to fit new gearbox ECU. Found a few websites that claim to be able to repair the ECU, best one seems to be www.BBA-Reman.com who quoted £169 +VAT + carriage. You do however need someone to take off and fit repaired ECU, had quotes ranging from £150 - £250 + £60 for 6L of new gearbox oil. However, I have been told by 3 independent garages that specialise in Audi's, BMW's & Merc's etc that they cannot be fixed, so I'm a little sceptical. To summarise the cheapest quote I've had for a new ECU fully fitted is from a garage on Tyneside (where I live) called Swift Motors 0191-2140056 who said they'd be able to do the lot for £860. Only bought car in February, absolutely love it, but sick as f**k at this unexpected cost which I can't afford. Had to take payment holiday from mortgage to cover the costs. Should have stuck with my old Ford Focus!! If anyone can offer any other tips I'd like to hear.

have you replace the transmission fuse it on the cigar lighter circuit?

It thinks you're kinky ;)

Seriously though, it suggests a problem with the Automatic Gear box ECU, or X/Y selector switch. Can also be water in the connector as it joins the gearbox, or someone's knocked the ECU which is under the passenger seat (RHD).
Sometimes this can be cleared by stopping, turning off the ignition, and starting again and it's fine. It's alerting you that it is in 'Limp Home' mode (3)

The flashing dash light is a warning about a failing system, most likely the fuel system. On mine it was the oxygen sensor on the way out. You need to do the diagnosis process with the ECU and this will report back the code that will tell you precisely what's the problem. You'll find the ECU diagnosis procedure here at the 300ZX online factory service manual.
Hope this helps.