I initially posted this in the wrong place as being a newbie it takes a little time to find my way around. My apologies.

I have a 2.1 litre petrol X-type that has developed a gearbox error message and is now stuck in limp mode.

Our local Jaguar garage has checked the wiring and says it is a error in the gearbox.. A nearby gearbox specialist says that it is not, and will be an electrical problem, leaving me very confused. I have been told that the problem may be the TCM, but nobody other than Jaguar themselves seems to have a matching TCM. Mine is 2x43-72401-JA but the only 2nd hand TCM I can locate has -JC instead of -JA. Is the difference significant? A new TCM seems to be £800+, and the car is only worth twice that, and I cannot be sure that it will solve the problem.
If I fitted the -JC would that matter or would it create problems?
I suppose the only way out is to buy the -JC TCM and try it, and if that does not work return it for a refund.

Any help more than welcomed as nobody in this area seems able to help

 

I know that many here on the forum despise AI answers on any kind of problem.
I, however, check, what AI has to say about it, and decide then by myself, if it makes sense or not.

In this case the AI answer is pretty much, what I would have written as well, thus it saves me the typing:

>>>>>>>>
For Jaguar vehicles, the P1643 diagnostic trouble code indicates a CAN ECM / TCM Network Malfunction, representing a communication fault between the Engine Control Module (ECM) and the Transmission Control Module (TCM). This issue is commonly associated with Jaguar X-Type, S-Type, XJ, and XK models.

Common symptoms include the Check Engine Light and Gearbox Fault Light illuminating, hesitation during acceleration, shifting issues, and restricted performance (such as limited throttle opening or being stuck in a specific gear). In severe cases, the vehicle may experience engine stalling or a no-start condition.

The most frequent causes are:

• Damaged wiring harnesses or corroded connectors between the ECM and TCM.
  
• A faulty Transmission Control Module (TCM), sometimes due to internal water damage.
  
• Low system voltage or a weak battery causing network instability.
  
• CAN bus circuit shorts or open circuits.
  

Repair typically involves inspecting and repairing the wiring harness, cleaning connectors, or replacing the TCM. Professional diagnostic equipment is often required to accurately troubleshoot the network communication failure.
>>>>>>>

THUS: If you go ahead and buy a new TCM (not sure where to get one), you are simply throwing money at the Jag without knowing, what's wrong.
As you can see above, in can be many things, and this includes many different expensive modules.
Note: Any swap of modules comes potentially with issues of non-compatibility - i.e. bigger problems.

I'd definitely start looking at those issues, which are cheap to fix, like:
> Damaged wiring harnesses or corroded connectors between the ECM and TCM.
i.e.: Check cables, check connectors, and don't forget to check ground connections! Check ground connections for corrosion.

And how's your battery doing?
> Low system voltage or a weak battery causing network instability.

It's hard to give an exact voltage, but if you measure anything below 12.7V with engine off on the battery, this might be cause to start wondering, if a low battery could be the root of all evil...
Also: Use at lease 2 different multi-meters to measure the voltage, as a multimeter, with its internal battery being low, could give you also wrong measurements of all kind...

And:
> A faulty Transmission Control Module (TCM), sometimes due to internal water damage.

Can you see any indication of water damage? Water coming thru the windshield?

 

Many thanks. Battery voltage is 12.3 V so that looks like a starter. I just need to find someone with a new battery to try. Now I find that my headlights do not work unless I pull back on the light stalk. I must admit that I am beginning to despair and want to replace the car, which would e a shame.

 

If you don't find a new battery, try a charger. I use CTEK chargers.

> Now I find that my headlights do not work unless I pull back on the light stalk.
Pulling on stalk should be high-beam!?
Sounds like your low beam does not work, just high beam?
Blown bulbs? Or fuses? Or defective relais?