Bob Laps,
Both!
It's against the law to tamper with the OEM Factory Emissions systems if you intend to drive on public roads.
Off Road, or Racing gets a pass on this, but that's not public roads.
It's against the law for a dealer or other shop that holds an emissions license to replace OEM Emissions control
hardware or software with other than the OEM design parts, unless the parts have been upgraded and approved.
Quite a few of the "tuners" have had to pay huge fines and are no longer in business.
Those who selll "defeat devices" like the DPF replacement pipes and SCR plumbing/wring/software can be fined and or
put out of business.
One wonders why so many folks want to add to environmental pollution.
Properly maintained Sprinters go hundreds of thousands of miles with the OEM Emissions controls working perfectly.
Roger