Check the quick connector (A) and set ring (B) for cracks or damage. If the connector and/or set ring are cracked or damaged, replace the connector.
Make sure the set ring is in place inside the quick connector. If the set ring is off the connector, replace the quick connector.
Replace a new O-ring (C) in the quick connector.
Check the lock (D). If the lock is damaged or deformed, replace it. When installing the new lock on the connector, push it straight down along the groove.
Clean the connecting surface of the thermostat cover (E), then apply clean engine coolant around the connecting surface.