My check engine light recently came on so I took it to a shop to see what caused it. The report I received didn't give a specific code, but instead, said L upper Emission System Oxygen sensor. Does anyone know which side the left sensor is? I know its before the catalytic converter but not sure...

Good Morning, So I am having issues with my check engine light coming on but first I will start off by explaining the background on the truck. I purchased the truck used from a dealer with all these mods already on it. It had AFE stage 2 intake, AFE inter cooler, AFE cold side Charge pipe, AFE...