The "Service Engine Soon" light came on steady two days ago on my 2004 GMC Envoy. I took it to AutoZone and their rep. said there was something wrong with the wiring/connection in the box because he couldn't get a reading. My Envoy had sit idle for an entire week at the airport in very cold temperatures (single digits) and rain/snow/ice while I was on a business trip. The light came on two days after it had sit idle.

Do I need to replace a fuse in the box and could this alone be causing the light to be on? (I have also wondered if AutoZone's equipment was defective.) Could there be a connection between my Envoy sitting idle for a week and the light coming on? I have tightened the gas cap. How long does it normally take for the light to re-set if the condition is temporary?

I have not noticed any changes since the light came on.

There equipment may have been defective I would try someone else to try to check them also the light does not reset itself you may try to reset the ecu just by unhooking the battery for about 15 to 20mins then see if it comes back on

Sometimes scan tools don't work I have a snap-on and a otc and sometimes one will work and the other won't it just the programs in the scan tool and the car programs not working together as far as the light sometimes sitting at idle the o2 sensors will stay rich or lean some cars and truck comp read the sensor info at 1000 to 1500 rpms that may be why the light came on I would say get the truck scaned by a shop and if it's a o2 code or a evap code have them clear it and drive the truck to see what happens or what code comes back