The Exhaust Gas Recirculation valve (EGR) is a common feature on diesel engines to reduce NO emissions by returning some of the exhaust gases to the combustion chamber, reducing temperatures and therefore NOx emissions. On older vehicles with higher mileages the EGR valve can become clogged up with carbon deposits, causing intermittent faults or for it to stop working completely. Using a premium fuel may help with this, as they containing cleaning agents which can reduce carbon build-up.