So far, I've never seen a link to a list of codes when someone asks what a particular code means, so I made one. I'll do the OBD-I codes later because there's 3 pages of those.
Code:
OBD-II Trouble Codes
Code Probable Cause
---- --------------
P0102 Mass Airflow (MAF) sensor circuit low input
P0103 Mass Airflow (MAF) sensor circuit high input
P0112 Intake Air Temperature (IAT) sensor circuit low input
P0113 Intake Air Temperature (IAT) sensor circuit high input
P0117 Electronic Coolant Temperature (ECT) sensor low input
P0118 Electronic Coolant Temperature (ECT) sensor high input
P0121 In range Throttle Position Sensor (TPS) fault (aka, problem with TPS reports)
P0122 Throttle Position Sensor (TPS) circuit low input
P0123 Throttle Position Sensor (TPS) circuit high input
P0131 Upstream heated O2 sensor circuit low voltage (Bank 1)
P0133 Upstream heated O2 sensor circuit slow response (Bank 1)
P0135 Upstream heated O2 sensor heater circuit fault (Bank 1)
P0136 Downstream heated O2 sensor fault (Bank 1)
P0141 Downstream heated O2 sensor heater circuit fault (Bank 1)
P0151 Upsteam heated O2 sensor circuit low voltage (Bank 2)
P0153 Upstream heated O2 sensor circuit slow response (Bank 2)
P0155 Upstream heated O2 sensor heater circuit fault (Bank 2)
P0156 Downstream heated O2 sensor fault (Bank 2)
P0161 Downstream heated O2 sensor heater circuit fault (Bank 2)
P0171 System Adaptive fuel too lean (Bank 1)
P0172 System Adaptive fuel too rich (Bank 1)
P0174 System Adaptive fuel too lean (Bank 2)
P0175 System Adaptive fuel too rich (Bank 2) (Said P0172 in Haynes)
P0191 Injector Pressure sensor system performance
P0192 Injector Pressure sensor circuit low input
P0193 Injector Pressure sensor circuit high input
P0301 Cylinder no. 1 misfire detected
P0302 Cylinder no. 2 misfire detected
P0303 Cylinder no. 3 misfire detected
P0304 Cylinder no. 4 misfire detected
P0305 Cylinder no. 5 misfire detected
P0306 Cylinder no. 6 misfire detected
P0307 Cylinder no. 7 misfire detected
P0308 Cylinder no. 8 misfire detected
P0325 Knock sensor circuit fault
P0326 Knock sensor circuit performance
P0351 Ignition coil no. 1 primary circuit fault
P0352 Ignition coil no. 2 primary circuit fault
P0353 Ignition coil no. 3 primary circuit fault
P0354 Ignition coil no. 4 primary circuit fault
P0355 Ignition coil no. 5 primary circuit fault
P0356 Ignition coil no. 6 primary circuit fault
P0357 Ignition coil no. 7 primary circuit fault
P0358 Ignition coil no. 8 primary circuit fault
P0400 EGR flow fault
P0401 EGR insufficient flow detected
P0402 EGR excessive flow detected
P0420 Catalyst system efficiency below threshold (Bank 1)
P0421 Catalyst system efficiency below threshold (Bank 1)
P0430 Catalyst system efficiency below threshold (Bank 2)
P0431 Catalyst system efficiency below threshold (Bank 2)
P0442 EVAP small leak detected
P0443 EVAP VMV circuit fault
P0452 EVAP fuel tank pressure sensor low input
P0453 EVAP fuel tank pressure sensor high input
P0500 VSS fault
P0505 IAC valve system fault
P0603 PCM Keep Alive Memory test error
P0605 PCM Read Only Memory test error