Carbon residue is the amount of carbon that is made by a chemical process, such as heating up oil. It is mainly considered to be a by-product of fuel. When gasoline is burned and used by a motor vehicle engine, it produces exhaust that contains carbon monoxide. Carbon residue is what the leftover particles of a fuel product are transformed into. There are tests that can determine the amount of residue that remains after certain fuels are heated or burned.

The test method that is used to calculate the amount of carbon residue is known as Ramsbottom Carbon Residue (RCR). This test is used to determine how much residue a fuel is likely to leave. It also helps calculate the fuel's tendency to combust or burn.

Residue can also be calculated as the Micro Carbon Residue (MCR) or the Conradson Carbon Residue (CCR). Both of these methods are equivalent in that they return the same numerical value. The test is conducted by placing a small amount of fuel oil into a glass container. As the glass is heated within a certain temperature range, the weight is calculated and subtracted from what the glass weighed prior to being heated.

High amounts of carbon residue can be damaging to the environment. For example, high levels of carbon monoxide can lead to an increase in the concentration of greenhouse gases in the atmosphere. These gases are thought to contribute to global warming — an overall increase in the average earth temperature over decades.

Carbon residues can also be life threatening or poisonous. Exposure to high amounts of carbon monoxide can lead to brain and cellular damage, including death by asphyxiation. It is considered to be a pollutant and certain fuels may result in higher amounts of carbon residue than others. Additives, such as ethanol, may be mixed into certain types of fuel to reduce the amount of carbon deposits.

Residues are usually only formed when the fuel oil reaches high temperatures. Machinery plants, vehicles and the majority of home heating systems all use some type of fossil fuel that produces a residue when heated. Some engine components may accumulate carbon deposits over time that may need to be periodically removed in order to maintain optimum performance. Chemicals such as fuel injection cleaners can help remove some of the deposits that may be the root cause behind engine stalling and rough starts.