the truck has about 8000 hours on it. I am using a handheld CO detector. Plugs, cap, and rotor are all in good shape. I did a little research and was able to adjust the air/fuel mixture. There is a little screw under a cap on the propane converter which i adjusted until the level was back down to.1% or lower. There were no codes. I was just doing the scheduled check and was very surprised when i saw the high numbers. Checked here and the toyota service manual, and decided to mess around until I got it back to normal. Thanks for all the input, hope I could or can help you too.

How many hours on this truck? What do the spark plugs look like???

Not sure why your getting much of anything out of the exhaust at idle with out any error codes? What are you using to check the percentage of CO? If you had a problem with any sensor you would have error codes. Without an on-board or hand held you will not be able to see any readings to make any adjustments. You may have to go to your local dealer and have them look at things and make adjustments if needed.