The article evaluates how well aftermarket fuel systems work for cars that run on natural gas or propane. Many of these cars were converted to use these fuels after they were bought. The quality of the conversion parts and installation can vary a lot, which raises concerns about how well these cars perform, how safe they are, and how much pollution they produce. There is more data on pollution from converted light-duty cars than from medium or heavy-duty ones, and there are worries about how much pollution these cars produce in real-world conditions. The report also looks at factors that can make pollution levels worse than what is usually reported. It also talks about issues like where the fuel comes from, what it's made of, how well the cars run, how safe they are, and what happens to the warranty.