There are dozens of trouble codes that can appear when engine lights are checked, which can vary from P0036 for a heater control issue to P0307 for a cylinder misfire. These codes that start with P0 refer to power train codes, which are not manufacturer specific but can apply to any engine. There are more generic codes that start with P2 and P3 as well and cover other parts of the engine outside of the power train.
One of the most common trouble codes is P0420, indicating catalyst system low efficiency. Other common trouble codes include P0171, meaning an issue with fuel system trim lean bank 1, and P0401, meaning that the exhaust gas recirculation flow is insufficient.
The OBD-II engines and codes are all emissions related. These codes can be triggered by any of the emission related features of an engine, including the catalytic converter, the fuel system and the ignition. The check engine light itself can only indicate a problem inside the engine. The only way to find out what the engine light is indicating is to hook up a scan tool to the vehicles diagnostic console. This is normally found on the underside of the steering column.
Not all code readers will give detailed information. Some will simply give out the code and nothing else, leaving someone to look up the code on their own. The more detailed code readers give the code, but also give users a one-line description so that they have an indication of what part of the engine or emissions system is causing the code to show up.