i will fill up the gas tank on monday and by wednesday its on a quarter of a tank. what causes this?
Do you have a CEL light on the dashboard? The place to start is the emission system, like the O2 sensors.
Which o2 sensor would it be the upstream or downstream
Change them all at the same time. In my experience if one go's your other ones are not far away from failing and you'll be back under the vehicle again.
BTW. Don't use the universal O2 sensors. They are crap! Use the same kind that came out of it.
Looking for a Used Envoy in your area?
CarGurus has 2,080 nationwide Envoy listings starting at $2,300.
Search GMC Envoy Questions
GMC Envoy Experts