1986 Other Nissan Models air care or etesting failed

Warranty problem
1986 Other Nissan Models 4 cyl Front Wheel Drive Manual 191km miles
----------------------------------------------------------------
Hello I just a 1986 Nissan Micra in absolute mint shape in and out, all new clutch and headgasket job ..the car just went in for aircare testing the car failed on CO by .44 a reading of 5.05 the max allowable is 4.61..on the idle test, can someone suggest something to pass it, should i maybe take it on a long highway run and then put ethanol blended fuel in it and then test it???or is it a o2sensor and how much are they are they costly to install..the vehicle also has a new catalytic converter so thats been changes as well..thanks Rick