Why does the service engine soon light come on 2001 Chevy Blazer LS 4WD?

When was the last time the oil was changed? My sister's minivan has that message displayed every 3 months. She takes it into the dealer and has the oil changed,which fixes it. There might be a faulty sensor sending a signal to the computer, causing the Service Engine Soon light Your Secondary Air Pump may be bad. Water gets into the pump and causes it to short out. If you get this replaced, make sure you get the breather vent extension kit installed or you'll have the same problem again in few months.