Don't know how scanner can tell re head gasket failure, compression test or fluid/gas migration the usual checks. Thermostat a common miscall in these situations
I'd bet on wp and cross fingers nothing else damaged. If car starts watch temp gauge and see if it goes past the usual position and keeps on going, headed for the red, in probably less than 5 minutes at idle, then shut off.
If wp check the FAQ section on home page for a how-to, metal impeller relacement preferred.
