DML FAQ : Exhaust : Should I replace both of my O2 sensors, or just one?
Some contaminants like anitfreeze will hose both O2 sensors. Otherwise, the rearward basically monitors the condition of the catalytic converter, while the forward one tells the PCM the A/F mixture. I believe the forward one is more likely to go bad. But if you are changing one, it's probably a good idea to change both (if the wallet allows). As for cleaning them, I believe that T/B or carb cleaner can be used. You can test them with an ohmmeter to see if they are still within working parameters. Per my 98 FSM, "disconnect the O2 Sensor connector. Connect the ohmmeter test leads across the white wire terminals. Resistance should be between 4.5 +/- .5 and 7 ohms. Replace the sensor if the ohmmeter displays an infinity (open) reading."