The "service engine soon" light on your 1998 GMC illuminates on the dash when there is regular maintenance that needs to be done. The GMC's ECM, or electronic control module, generates a fault code or a trouble code to indicate that maintenance needs to be done on the vehicle. When you've discovered the source of the problem by doing a diagnostic scan of the computer (these can be done for free at most auto parts stores), you can then fix whatever is wrong with your GMC. The last step is to reset the "service engine soon" light.
Open the hood of the GMC and loosen the retaining nut on the negative battery cable clamp.
Slide the cable clamp off the negative battery terminal.
Wait 15 minutes, then reconnect the cable clamp.
Start the engine and run it for 5 to 10 minutes to allow your GMC vehicle to relearn its idle procedure.
