Cars need to be driven. Sometimes hard. "Blowing out the carbon" is sometimes a useful thing.
24K miles on a 6 year old car is not enough. You probably drive short miles like 5-10 minutes and then shut down. This doesn't really allow the engine and systems to get up to operating temperature. So you can get sludge in the oil, deposits in the engine and on smog sensors etc. Just because the water temp guage reads normal doesn't mean the engine and sensors are up to proper temperature.
On short trips the engine runs rich which will cause deposits on the O2 sensors and the cats. You need to drive at least 20 minutes at freeway speed to get things up to temp.
My guess is you have deposits on the O2 sensors due to inadequate temperatures.
Maybe you can "burn" them off with a nice couple hour freeway drive, with a few good hard blasts at full throttle. Other sites I have visited talk about using a propane torch on the O2 sensor to clean off the part, of course the sensor has to be out of the car to do this. The least risky approach is to just replace the sensor and drive longer trips at least once a month or more.
Good Luck,
Jim Henderson
