1997 gmc jimmy e-test problem

i have a 1997 gmc jimmy 4,3 340,000KM and for som reson i got HIGH carben or CO% on the test maken it fail. so far i have changed the wires, plugs, air and fuel filters, oil change 500KM ago, someone has said to try methelhydrat in the fuel the day of the test, ive failed 2 tests so far so i dont want to fail a nother. i did take it for a nice run the day of the test to get it nice and hot. but was told the methalhydrate will make it run hotter... does anyone know where else i can look to maybe get a idea. theres no engine light on. but the truck has thrown off O2 sencer a fue times. anyways please help my sticker is due by the end of this month and i need my truck to get to work... thanks:runaround::runaround::runaround:

Try the EGR Valve.

Egr only affects Nox. You are running rich, bad injectors, high fuel pressure, bad fuel regulator ( common), o2 sensor, these things need to be checked out by a qualified technician with the proper equipment. A full scan should reveal the problem. Watch for cat efficiency from the scanner readings also, which is a comparison of the pre and post cat o2 inputs.