I have a JVC KW-AVX840 and the touch screen stopped working today. The unit still works, via my steering wheel controls any idea how to get the screen to respond to touch again?

See page 77 troubleshooting of user manual for help
B5A-2488-00_KWV_M_EN.pdf (jvckenwood.com)

It is too cold for it to work properly. Check the operating temperature range in your user manual.

http://www.usersmanualguide.com/

Probably a blown output amp.

This is not a question for JVC but more for GMC. On the JVC radio side, if it can be controlled via steering wheel, you will see a blue with yellow stripe cable that states [STEERING WHEEL]. Not all radios can adapt to steering wheels controls.

Sounds very strange ... I have the exact same set. Try removing the Bluetooth dongle and re installing it again and see if this helps. Is this a new install? You r ground is good right? to the frame not just a black wire from old stereo you replaced?

is there a diagnostic setting for touchscreen calibration. Could have gone bad. you are probably still in warranty, take that bad boy back

you may need to reset the unit. after that do a screen recalibration. if that doesn't work the ribbon cable could be broke

Are you attaching it via usb on the deck or aux? Because if it is via usb the touch is probably not supported.

you need a pc99-x06 and then a patch lead which is stereo specific, so if you buy a JVC it will be pc99-jvc, alpine will be pc99-alp etc etc for your steering wheel controls (these are made by a company called autoleads who do loads of different stuff for car audio)

the stereo you purchase will have to be steering wheel control compatible, which should be stated in the description/instructions

you can have a sub amp etc with steering wheel controls, as thats run from the stereo and main wiring loom, and isn't affected by your steering wheel controls