Troubleshooting codes for GM vehicles include Pxxxx for powertrain, Bxxxx for body, Cxxxx for chassis and Uxxxx for network communication errors, where X stands in for various digits. If the second digit is 0, the code is generic or global; if it's not, the code is unique to the particular vehicle. The third digit specifies the category of the problem, while the last two digits refer to the exact problem. The full list of GM codes is available at Trouble-codes.com.
The third digit can be 1 or 2 for issues related to the measurement of air and fuel, 3 for the ignition system, 4 for additional emission control, 5 for speed and idle regulation, 6 for computer and output signals, 7 or 8 for transmission and 9 for control modules, input and/or output signals.
P0127 indicates that the intake air temperature is too high. P0606 indicates a powertrain control module processor fault. U0215 indicates a loss of communication with the door switch A. U0317 is displayed when there is a software incompatibility issue with parking brake control mode. P1814 means the torque control is overstressed. B2725 signals that the active transfer case mode switch is malfunctioning. P2560 indicates that the engine coolant level is low.