my buddy has a 99 f150 knocking real bad the check engine light came on gave me two codes both were po306 any help would be appreciated thanks has the 4.2 liter

 

It is "Ho2s heater control circuit (bank 1, sensor 2). Replace the O2 sensor and you should be ok. Good luck.

 

P0306 is for a cylinder misfire on number 6 cylinder. Since you said it is the 4.2L V6, it is the last spark plug on the drivers side. Might be a good time to change all the plugs and wires or Coil boots(whichever it has). Not sure when the COP change over was on the truck. The O2 heater code for the one you mentioned is P0141.

 

Yeah... your right! i was looking at the wrong code. Like he said, ide change your plugs and wires, and maybe the rotor and cap on the distributor. Sry about that... good luck.