Rich Benoit, a YouTuber and a rogue Tesla mechanic, set out to convert a Tesla with a destroyed power train into a diesel vehicle. The Environmental Protection Agency (EPA) told him to stop.

Benoit got the letter from the EPA after posting several videos about conversions using a diesel engine. In one, he's driving an old bread truck around town while it spews black smoke out of its exhaust. In another, he's pulling the engine out of that antique truck and putting it into a Tesla with a totaled engine. The EPA's warning wasn't specific, but Benoit was pretty sure one of these two videos got him in trouble. 

According to Benoit's video about the letter, the problem is that the specific engine he's using is an antique. "This is a pre-emissions diesel," Benoit said in the video. "There's no emissions devices on board in the truck."

Diesel engines can spew a lot of toxic emissions into the air and were a huge contributing factor of pollution in the United States before the EPA started regulating it. The standards are complicated and vary state to state, but basically cars manufactured after a certain period require devices in the car that phase out harmful particulate matter found in diesel emissions.