I did mention a tune up to the customer but they dismissed the idea because the problem started so suddenly. I connected a co analyzer after changing O2 sensor. I had readings going up & down from.5 % to 2 %. I thought that was probably normal to fluctuate but I'm not familiar with the system. I wish I had taken a reading before for a base line.The complaint was truck set off a co detector in an office connected to the building they run the truck in.I'm going to push the tune up again.Can I just lean out the mixture by removing cap from screw on the mixer?