Find out immediately if your diesel fuel is fresh, marginal, bad or contaminated with Diesel Mechanic In A Bottle 4 in 1 Test Swabs from B3C Fuel Solutions. In 3 minutes, the diagnostic tool helps you identify oxidation, water contamination, rust and scale, and sludge/asphaltenes - all indicators of possible water performance hits and other contaminants, filter filters plugged particles, dirty injectors, frozen or gelled fuel filters, injector pump failures, and fuel filter sludge. Diesel fuel tester.