Auto manufacturers standardized the diagnostics and the ports to access them (OBD) and tractor manufactures could have followed suit, it's all there and technically available, but they didn't and for a reason. They did what they did in the name of profit after the sale.
Sure is nice to be able to plug in my scan tool and and diagnose the DTC's (trouble codes) on any of my vehicles, don't matter the make, we own Fords and GM's.
Just did it yesterday to my Focus, had a 'check engine light', plugged in the scan tool, downloaded the code (front O2 sensor rich condition) and corrected it and cleared the code. I had run a can of Seafoam intake cleaner through the motor (it's a Gasoline direct injection motor and GFI engines are noted for intake valve deposits) so the can of Seafoam cause a rich mixture condition on the lead O2 sensor (I knew it would) and the sensor threw a check engine code and illuminated the check engine light.
You get a check engine light on a new T4 final tractor, there is nothing as the end user you can do but call an authorized dealer and then pay for a service call so the tech can come out and do what I did in the driveway, diagnose the issue and take corrective action if necessary and in my opinion it's 110% BS and all for corporate profit and why I will NEVER own a T4 final tractor.
Sure, manufacturers under Federal EPA laws have to warrant emissions systems and related components for 5 years and all well and good but what about after the 5 years is up? You are on the hook for everything and I'm here to tell you that T4 final diesel engines are failure prone as they get older. The Freightliner dealership I retired from, a full 80% of their after sale shop work is emissions related, either electronic or component issues.
I don't sell off a tractor when it's 4.5 years old because of emissions issues, I keep them a long time and that isn't prudent with the new T4 final engines and..... with the incoming new President and his buddies with their 'Green New Deal' bs, I suspect it will only get worse, much worse.
Have at it but expect to pay for 'clean air' in the end and pay dearly. You can always buy an electric tractor. I see JD has one out now. 6 hour duty cycle time for light farm chores (no ground engagement) and it's only 125 grand.
Farmer down the road from me (we are good friends) has about 3 million in JD rolling stock and 90% of them are T4 final. His comment to me the other day was, 'You don't ever own a John Deer even though you pay for it, you buy the right to use one'. He cannot do anything to any of those units. The units 'ralk' to the dealer via the Green Star on board GPS satellite system and 'tell the dealer (not him) if there is a pending issue. it's so bad that when it's oil change time, he must use JD filters and lubricants because the filters have sensors in them that monitor oil life, viscosity and contaminant level and when it's time, the filters throw a code that the Green Star sends to the dealer and the dealer notifies him it's time for service. All BS and all about the long dollar. I get to witness that close up in his shop.
When they run, they are peachy. When they don't all hell breaks loose.