Good info on warming up the exhaust. I actually run my gennys hard for a little bit each time that I use them, which should help burn the carbon out. Although I don't have a fuel injected unit as the above. (old Honda em5000 and a new eu2200) The OP says that the 02 Sensor isn't covered, but what is really meant I think, is that the owner will need to pay for the service done, the cleaning of the 02, and the replacement of the sparkplug etc,. I would suggest that the OP ask the mechanic what was used to clean the 02 sensor and maybe purchase that cleaner so that he can perform it himself if he ever needed/wanted to do it himself later.