1999 Land Rover OBD2 code

I went out and bought a code reader having a landrover and all you need one. So I ran a code check and came up with 2 codes the priority code was P0133 - 02 circuit slow response (bank 1, sensor1)
Second code was P1170
landrover Lambda sensor aging tv bank 1 drive cycle C: signal out of range - above maximum/below minimum?
Please let me know what I need to do to get this fixed. I took it in to a mechanic before I bought the code reader and he said that he ran codes and came up with the head gaskets need to be replaced. Thanks
Louie