2003 Pontiac Montana Owner's Manual - Page 220
2003 Pontiac Montana Manual
Page 220 highlights
The SERVICE ENGINE SOON CHECK ENGINE light or comes onto indicate thatthere is a problem andservice is required. Malfunctionsoften will be indicated by the system before any problem is apparent. This may prevent more serious damage toyour vehicle. This system is also designed to assist yourservice technician in correctly diagnosing any malfunction. Notice: If you keep driving your vehicle with this light on, after a while, your emission controls may not work as well, your fuel economy may not be as good and your engine may not run as smoothly. This could lead to costly repairs that may not be covered by your warranty. Notice: Modifications made to the engine, transaxle, exhaust, intake or fuel system of your vehicle or the replacement of the original tires with other than those of the same Tire Performance Criteria (TPC) can affect your vehicle's emission controls and may cause this light to come on. Modifications to these systems could lead to costly repairs not covered by your warranty. This may also result in a failure to pass a required Emission Inspection/Maintenance test. 3-43