1997 Mazda 626 check engine light

1997 Mazda 626 ES mileage: 128,000. My car recently tripped the engine check light. My wife took the car to our local Mazda dealer (I was out of town) and they pulled codes 23 and 24 from the computer. According to the service manager, those codes related to high idle and misfire. They adjusted the base idle, perform a top end carbon cleanout, and reset the computer. According to the service report, they road tested the car for about nine miles and the check light stayed out. When my wife picked up the car, she drove it a short distance away from the dealer and, as you might expect, the check light came on again. I'm perplexed. I did a bit of Internet research yesterday after the car visited the dealer and my information indicates that codes 23 & 24 relate to the right side oxygen sensor, not high idle and misfire. All the various engine sensors are original. Also, plugs are new NGKs and we do use high octane gas as recommended by Mazda. My speculation is that the oxygen sensor is bad. With 128,000 miles on the car, it appears prudent to me to replace both the right hand and left hand sensors. Am I heading in the right direction?