: 01 STS gas mileage problems

02-26-08, 08:54 PM
Recently purchased a 2001 STS with 80k miles. Car is in excellent condition in and out. Factory 17 inch wheels, runs and drives very nice. Info I have found indicates highway mileage is 27 mpg average. Have changed plugs, and air cleaner, best mileage to date is about 22.5 with cruse set at 74. I do see the occasional P0420 code indicating 0s sensor or cat converter. Any suggestion?