My s-type has 66k miles. Gearbox fault light came on and is stuck in 3rd or 4th gear. I had codes cleared, then came back on after 7-10 days. Came on in the morning, I stopped to get gas and it did not come back on for a few more days. I just had the codes reset again. Below are the codes I am getting. Can anyone diagnose what the fix is? Thanks for your help.

P0736 Incorrect gear R ratio
P0730 Incorrect gear ratio
P0731 1st gear ratio incorrect

 

First things first. Use a voltmeter to check the available voltage across the battery terminals with the ignition in the OFF position. There should be a minimum of 12.6 volts available for all the modules to function correctly.

If the voltmeter shows less than 12.6 volts, charge the battery for three to five hours at 2 to 5 amps and retest. If the voltmeter still indicates less than 12.6 volts, replace the battery.

Many random DTCs and instrument cluster warnings are the result of a failing or weak battery.

 

Unfortunately, the POSSIBLE CAUSES for all the DTCs you posted are Transmission mechanical failure.

 

As posted above you might be in for a rebuild.
But lets do some simple stuff first?
Have you checked the fluid level?
How does the fluid look?
.
.
.

 

Also, if you are up for having a go, to see if it helps, swap the 4 tubes and the adapter valve body (=bridge seal) - see in my thread the entries after 1st of Dec. 2022:
https://www.jaguarforums.com/forum/s...-264102/page3/

And do a full flush of ATF at the same time - or not, if you have little confidence that this might some the issue and want to save the money for new ZF Lifeguard 6...

 

The OP has not indicated his location, has not completed his "intro" in the New Members forum and has not indicated whether his automatic transmission has ever been serviced; at 66K miles there is a good chance it has not...

I guess he figures we all have a side gig working for the "Psychic Friends Network"...