This is a problem with the EGR System (Exhaust Gas Recirculation). The engine module is detecting that the EGR Valve is not closing properly. You may be experiencing a rough idle if this is the case. This is commonly caused by carbon deposits in the valve and EGR ports. This carbon will need to be removed and the system tested for verification. If the problem still occurs, then I would take it to a high quality repair shop and have them look at the technical service bulletins from GM. 97-T-20A: MIL (Service Engine Soon Telltale Lamp) On and EGR DTCs 97-T-20: For EGR Diagnostic Trouble Codes These bulletins have advanced testing procedures specifically for these trouble codes, and special software may be needed to fix the problem. Good Luck!