When the Engine Management light or MIL light (Malfunction Indicator Lamp) comes on it can be difficult to diagnose the problem without a scan tool. Code readers can cost from £30-£4000, it depends what you want to do as to how much you need to spend. They all use a standard socket after 2001 although some will have had the same socket fitted since 1996 see picture
EOBD (European On-Board Diagnostics) SOCKET
They will all read
Diagnostic Trouble Codes - displays the pending and stored Diagnostic Trouble Codes (DTC) logged by the vehicle and the Freeze Frame. If the vehicle’s Engine Management light has or is on there will be a stored code.
Only the better ones will read ABS, Transmission codes and reset service lights.
Some might display
Live Data - displays real-time sensor measurements in a graphical and meter format. This can be useful for checking for RPM in a non-start, if it reads RPM you know the Crankshaft Position Sensor is working for example.
Some are capable of
Reprogramming- This can be useful if there is a software bug that has now been fixed.
Manufacturer Specific- These may communicate easier but if you change your car to a different make will probably not work.
There are two types of code reader
PC based usually will do more for money if you already have a Laptop.
Stand-Alone tools- these are hand held and of rugged designs, intended for use in and around cars (i.e. no lugging a laptop in and around a car).