Every car built since 1996 in the United States, and since 2001 for petrol and 2004 for diesel in Europe, stores fault codes in a format that is the same on a Fiat and on a Ford. 12 of them are explained here in plain words, with what the repair actually costs in four countries and what the code means if you are the one buying the car.
Codes from the systems that control what leaves the tailpipe: catalytic converter, oxygen sensors, EGR, EVAP and the diesel particulate filter.
Codes from combustion, fuelling, air intake and timing. This group contains most of the faults that are expensive to ignore.
The letter says which part of the car raised the code: P powertrain, C chassis, B body, U network.
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