WHY IS COOLANT LIGHT COMING ON AFTER TANK CLEANING AND NEW CAP ON TANK????? on 1998 Chevrolet Malibu

AS STATED ABOVE, TRIED TO CLEAN SLUDGE FROM INSIDE TANK SENSOR AND REPLACED THE TANK PRESSURE CAP. LIGHT ON DASH CONTINUES TO COME ON, 1998 MALIBU, SIX CYL. ALSO, CHECKED FOR ANTI F ON DIP STICK, COULD NOT DETECT. ANY SUGGESTIONS

Asked by for the 1998 Chevrolet Malibu
1 answer
I would check the sensor to start with what color was the sludge that you took out of the rad?