When does the ODB throw a CEL? Specificially, if the O2 sensors are bad (any of the 4), does it throw a CEL? Or, does it just manifest itself as poor mileage? Also, does a failing MAF cause a CEL to be thrown as well? Just curious under what parameters these occur.

 

When the voltage stops changing on the O2 sensors it throws a code. If you watch an O2 sensor live with a scanner a good one will have the voltage jump all over the place as the O2 conc. changes but a bad sensor will stay at one voltage or respond very slowly.

 

maybe I should ask it this way... I am thinking I should be getting better mileage.... I am getting around 15 ish mpg.... but I wondered if something could be contributing to that without throwing a CEL code....

 

How many miles on your truck, have you replaced your factory plug wires, what octane gas do you use, what air pressure in your tires? All these will contribute to your mileage and 15 is not bad, many get a lot worse.

 

76K ish on the truck. I put magnecor 8mm on it, and use 93 octane always.. Tire pressue... usually I run at 40 front, 45 rear