What is DEF?
DEF or Diesel Exhaust Fluid is used for emissions control by modern diesel engines. It is injected into the exhaust system to reduce and remove harmful NOX emissions by breaking down the gasses into nitrogen and water.
Why is it Needed?
The vehicle must be operated with DEF if the exhaust gas after-treatment system is to function correctly. If you operate the vehicle without DEF or with emissions-relevant malfunctions, the legal MOT approval is invalidated. The vehicle could be prohibited from public road use by an official ordinance.
Engine management monitors the exhaust gas after-treatment components for compliance with emissions laws and regulations. If you attempt to operate the vehicle without DEF, with diluted DEF, or with a different reducing agent, this will be detected by the engine management system.
Other emissions-relevant malfunctions, e.g. metering malfunctions or sensor errors, are also detected and logged. The engine management system subsequently prevents the engine from being restarted after issuing a warning message. Therefore, refill the DEF tank regularly during vehicle operation or, at the latest, after receiving the first warning message via the instrument cluster.