Check Engine Light Stays on since purchase of 02 kia sedona with 89,000 mileage.

FIRST- oil light is for PRESSURE NOT VOLUME. A motor can have one quart of oil in it and still have great oil pressure at idle. Second, seller said it was just changed , did you or somebody with you verify this visually? Did you top-off oil after discovering it was low , to see if this helped the knocking noise? Last- how many miles have you driven it under these conditions? Check engine light is probably on due to a valve control problem related to the low/sludged oil.