The NYTimes is running the following article. Apparently an draft EPA report was leaked by staff. The report says diesel tuners are increasing nitrous oxide emissions. Buried in the article is this nugget: the devices are not detected because most states don't do tailpipe detection, they rely on the vehicle's computer to detect emissions problems. If the computer has been modified then the emissions are not reported.
Illegal Tampering by Diesel Pickup Owners Is Worsening Pollution, E.P.A. Says (Published 2020)
An E.P.A. investigation has found that the owners of more than half a million diesel pickup trucks have installed devices to defeat emissions controls and boost pollution.
www.nytimes.com