It depends on what kind of car/truck it is.
On a Chevy it's the Camshaft Sensor is not functioning properly.
Ford it would be lean condition that could be caused by several different problems.
Trouble code P0304 means:Cylinder 4 misfire detected
Trouble code P0301 means:Cylinder 1 misfire detected
Trouble code P0134 means:O2 sensor circuit no activity detected (Bank 1 Sensor 1)
Trouble code P0171 means System too lean (bank 1)
Trouble code P0325 means:Knock sensor 1 circuit malfunction
For 1996 and newer Trouble code P0170 means:Fuel trim malfunction (bank 1)
Trouble code P0301 means:Cylinder 1 misfire detected
Trouble code P0491 means:Secondary air injection system (Bank 1)
Trouble code P0075 means:Intake valve control solenoid circuit (Bank 1)
For 1996 and newer Trouble code P0030 means:HO2S heater control circuit (bank 1 sensor 1) Trouble code P0431 means:Warm up catalyst efficiency below threshold (bank
Trouble code P0327 means: Knock sensor #1 circuit low input (Bank 1 or single sensor)
Trouble code P0131 means: O2 Circuit Low Voltage (Bank 1, Sensor 1)