John, oil burning will give high CO, as will sump oil contaminated with fuel. Have you checked the emissions with the breather disconnected to see how much they are contributing? A very accurate way of testing the head gasket is to put the gas analyser probe into the air space above the coolant in the expansion tank. Obviously be careful not to suck up any coolant. There should be no HC's. Any reading at all can only come from the combustion gases entering the cooling system. HTH, Peter