I took my car in for emission testing in colorado and they failed me because they said i needed 4 02 sensors. they could only find 2. i have never did any mods to my vehicle. it has always passed before. the guy barely spoke english and looked under my car for 30 minutes. Are their 4 02 sensors on the vehicle from the factory. if so where are they. if not i guess i need to add 2 more? how do you do that please help