Found out this problem from engine code analyzer: P0325 Knock Sensor 1 Circuit Malfunction. Mechanic explained that engine manifolds had to be removed to access/replace the sensor (is that true?) which required a full day of work and cost me $1550! I was told that it controls the ignition timing and could cause engine damage if left un-repaired for a long time. Part of the cost was, once they removed the manifolds, they found a coolant leak into two cylinders and had to fix that too, the mechanic claimed.