have the mechanic check for errors via the obd port. replace the o2 sensor as well.

Try to ask around any of your friends who have a standard obd2 scanner. Read what are the problems and if possible reset the error and run again to see if the same error still prompts up. That should help you validate the problems before running to the SC which may chop your $$.

Common problems are, faulty o2 sensor, dirty MAF or MAP sensors which results in wrong parameters. The ECU will inject more fuel or hop into safe mode to purge out more fuel to avoid lean combustion.

Also, if you have a strong nose, try to smell whether there's sweet smell out of the fume. Most extreme cases you can see unburn petrol being cough out of the exhaust.