Auto trouble codes (or fault codes) are codes that are stored by the on-board computer diagnostic system.
These are stored in response to a problem found in the car by the system.
These codes are stored when a sensor in the car reports a reading that is outside the normal/accepted range
The codes identify a particular problem area and are intended to provide the technician with a guide as to where a fault might be occurring within a car. The codes should be used in conjunction with the vehicle's service manual to discover which systems, circuits or components should be tested to fully diagnose the fault.
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P for Powertrain which means the engine, transmission, emissions, and ignition to name a few. The codes associated with P is the largest set of codes.
B for Body includes items such as power seating and airbags.
C for Chassis which covers the anti-lock braking system (or ABS), axles, and brake fluid.
U for Undefined, meaning that any other aspect of the car is going to be found in this system.
The first digit in the code will tell you if the code is a generic or manufacturer specific code.
Codes starting with 0 as the first digit are generic or global codes. It means that they are adopted by all cars that follow the OBD2 standard. These codes are common enough across most manufacturers so that a common code and fault message could be assigned.
Codes starting with 1 as the first digit are manufacturer specific or enhanced codes. It means that these codes are unique to a specific car make or model. These fault codes will not be used generally by a majority of the manufacturers.
The first digit might be also 2 or 3. In this case the type depends on the system. B2xxx and C2xxx codes are manufacturer controlled while B3xxx and C3xxx codes are reserved at the moment. P2xxx codes are generic codes while P3xxx codes are manufacturer controlled. U2xxx codes are manufacturer controller as well as U3xxx codes.
There are two types of obd codes (DTC) that apply to OBD II. I have them listed below with Type 1 being the more important because they can cause quick and severe damage. Type 2 being not quite as important but you still need to deal with it.