The “Service Engine Soon” light indicates that there is a problem with the operation of the Fuel, Ignition or Emission Systems. This light may come on if you have made modifications to the engine, transmission, exhaust, intake, fuel system or the replacement of the tires with others that are not the same TPC (Tire Performance Criteria).
Light is Flashing: is caused by a misfire; this increases your vehicle emissions.
• Turn the key off, wait 20 seconds and restart the engine, if the light still flashing take you car to a mechanic shop as soon as possible.
Light is On Steady: there is an emission system problem.
• Make sure the fuel cap is not loose and properly installed.
• Electrical system may be wet, when dries out the light should turn off.
• Changing brands of fuel, it will require at least one full tank of quality fuel to turn the light off.
Was the "Service Engine Soon" light on when the GMC was brought to the dealership? The on board computer on your Envoy is constantly checking sensors and emissions for performance, opens and shorts in electrical sensors and wiring. If it sees a problem it holds it in its memory as a pending fault until it sees the problem again under the same circumstances, it then turns on the light. If the problem corrects itself the computer sees that it no longer has a problem, looks for the same exact drive cycle to be completed in which it first saw the problem and may turn of the light and erase the code itself as it no longer sees a problem. If the Check Engine/Service Engine Soon Light is on when you bring in your Envoy there should be no reason for a garage not to able to read a trouble code unless the scan tool is not compatible (a dealer better be using a GM compatible scan tool) or the diagnostic socket has a problem not allowing the scantool to connect to your Envoy's computer.