How many CMP sensors are there on my car and where are they located?

My HHR check engine light has come on and the trouble code is PO341. I recently replaced the plugs, coil and ignition module. After I did that I got a trouble code PO420. I put additive in the gas and now I have the 341 code. The 420 code has gone. I have read that the CMP sensor may be bad but can't find out where they are, even though I know what they look like also someone told me there is only one, Hence my question. I am hoping for an inexpensive solution. Thank you, Merib