"It is a crime to knowingly falsify, tamper with, render inaccurate, or fail to install any "monitoring device or method" required under the Clean Air Act, including a vehicle’s on-board diagnostic system. Clean Air Act section 113(c)(2)(C)." https://www.epa.gov/sites/default/files/2020-12/documents/ta...

EPA defines tampering here:

Tampering. You may not remove or render inoperative any device or element of design installed on or in engines/equipment in compliance with the regulations prior to its sale and delivery to the ultimate purchaser.

https://www.ecfr.gov/current/title-40/part-1068/section-1068...

It seems primarily about bypassing or disabling emission controls, not user controlled fuel mapping, or mods like putting in a performance air filter or exhaust. But EPA does consider a flex fuel conversion tampering.

https://afdc.energy.gov/bulletins/technology_bulletin_0807.h...

The EPA has a document here https://www.epa.gov/sites/default/files/2020-12/documents/ta...

With a relevant paragraph on page 2

The site you linked mentions the carve out that the EPA has, but note that it requires both retaining or beating original behavior and requires extensive prod of that fact. A similar law affecting phone home circuits would almost certainly not find disabling the ability to phone home as in compliance.

The part of your post that made me curious was whether fuel mapping, or ECU swapping was illegal. It looks like it is in a grey area under Clean Air Act, but generally interpreted as legal as long as you aren't doing things to make your emissions worse.

At least as far as ECUs go, almost every after market ECU I’ve seen doesn’t control OBD II or the CEL (or does so very minimally) and is therefore immediately in violation of not conforming to the requirements to retain OEM level behavior. Fuel mapping is more grey, largely due to the ability of some OEM ECUs to be reflashed and thus retain OBD behavior.

Title 2 of the Clean Air Act "authorizes the EPA to set standards applicable to emissions... the CAA prohibits tampering with emissions controls, as well as manufacturing, selling, and installing aftermarket devices intended to defeat those controls."

They just got a $10M civil judgement against a couple "diesel tuners" here in Michigan:

https://www.justice.gov/opa/pr/united-states-awards-10-milli...

https://www.epa.gov/enforcement/national-enforcement-and-com...

but yeah, this is civil action against vendors, not anything that police will fine people for on the side of the road.