Many common OBD2 codes are emissions-system related, including P0420, which is the code for low efficiency in the catalyst system. P0171 indicates a lean condition in Bank 1. P0174 indicates the same problem in Bank 2. P0401 is also common, and indicates the Exhaust Gas Recirculation valve has insufficient flow.
OBD2 codes related to the Evaporative Emission Control system are common. EVAP-related codes are P0411, P0440, P0442, P0446, and P0455. They accounted for the most common group of trouble codes (24.3 percent) found during testing of cars by the Illinois Environmental Protection Agency during OBD II plug-in emissions testing in the Chicago metro area.
Engine misfire trouble codes P0300, P0301, P0302, P0303, P0304, and P0305 are also common. While the codes do not specifically indicate what is causing the problem, they are generated by the crankshaft position sensor when it detects a slight loss of speed in the rotating crankshaft.
Catalytic converter codes P0420 and P0430 are commonly experienced, and accounted for the fourth highest number of codes seen in the Illinois EPA testing. Oxygen sensor trouble codes P0133, P0135, and P0141 are common. OBD2 code P0128 is common, and indicates coolant thermostat trouble. P0325 is common and signifies a knock sensor issue.