ECUs - Electronic Control Units

From the late 1980s ECUs (Electronic Control Unit) were introduced to many vehicles. Today pretty much every petrol engined car has an ECU and in the last 5 years or so diesels now also have ECUs. Sometimes the ECU is referred to as the computer or brain. Other parts of the vehicle, like the braking system, may also have a dedicated ECU for it's operation but the following will only deal with the engine ECU which is the main control unit you'll come across with regards to the operation of a modern vehicle.

What does the ECU do?
The ECU monitors several parts of the engine system, especially the fuel injection elements and adjusts the control of the injection system to provide optimum performance. If any element should become faulty the ECU can alter settings so the engine can keep on running until the fault can be attended to - called 'Limp Home Mode'.

The ECU also has a memory which records the faults as codes. This makes fault diagnosis much easier for vehicle technicians.

The codes are read by your dealer through relatively expensive garage equipment but can also be read by the DIY users with either a cheap handheld device available from your local motor factors or with a home fabricated piece of wire.

DIY Code Reading
The following should work on most Peugeot models that use the same diagnostic socket. Once the correct connections are made the codes will be signalled by the ECU light illuminating on your dashboard. Your ECU fault light should look something like this (the 'check engine' lettering may not be present). See here for full details