Fuel Systems: Emission Control: On-board diagnostics
OBD systems
 
On-Board Diagnostic systems use the vehicle’s computers to detect problems with its emission components and other systems.
 
Diagnostic trouble codes
 
The diagnostic trouble codes inform the technician of the computers opinion of the location of a system fault.
 
Monitoring emissions
 
OBD II systems monitor the emission systems in a vehicle either ‘continuously’ or ‘non-continuously’.