Check the coolant fluid level on a cold engine. If it is low, especially if you have a coolant leak, you might have found the problem right away. Otherwise, start with checking the fuses #1 and #4 on Series II models or #5 on the older ones. If they are OK, put your finger on the leftmost relay behind the fuses; when you turn the ignition on (don't start the engine), it should click. If it doesn't, replace it.
You can delay the corrosion if you use stainless steel exhaust components. For instance, Walker, a widely known aftermarket manufacturer of exhaust parts has a family named Aluminox. These parts are made of stainless steel and covered by a layer of aluminum alloy. The estimated life of these exhaust components exceeds five years under normal use while their price is only about 30% higher than that of the standard components. The last time I asked it was only available for the diesel models.
There are basically two electrical measurements used in testing the various components. Voltage tests should be carried out with the ECU connector connected (unless otherwise stated in the test description). The cleanest method is to use a Break Out Box (or BOB), which is an extension connector placed between the ECU connector and its plug, making the various terminals accessible for measurements. If no BOB is available, use backprobing: peel back the insulating boot from the plug and attach the probe to the relevant pin.
Citroën used various systems of several manufacturers. On gasoline engines:
The components of the various systems and their interoperation can be studied on the following diagrams:
Citroën used various systems of two manufacturers, Bosch and Magneti Marelli. All those systems operate on very similar principles so troubleshooting them involves more or less the same steps.
There are two basic categories: fuel injection (EFI) and engine management (EMS). EFI systems, as their name implies, are responsible for the injection of fuel only, the ignition sparks are created using traditional methods (a breakerless distributor). EMS systems, in contrast, govern both fuel injection and ignition themselves.
Citroën used Weber and Solex carburetors:
Citroën specifies the following filters:
Although theoretically this might be a failure of either the meter or the level transmitter, the second one is much more likely and, first of all, much easier to check (to get at the meter you would need to remove and disassemble the instrument panel).
As a general rule, older engines (up to 1987 approximately) are not explicitly designed to run on unleaded fuel. Later engines are capable if the necessary adjustments are made. A few engine models are capable of running on both kinds of fuel without any adjustment. And finally, every car equipped with a catalytic converter, either in the factory or with an aftermarket product, must be run on unleaded fuel only.