Can the radiator fan run in auto mode? If yes, at Slow speed or Full speed? If not, either the coolant sensor is shot or the internal circuitry of the ECM attached to the coolant sensor is faulty. There is a possibility that the main connector connected to ecm need some cleaning, just buy a can of 7CF carb cleaner + WD 40, remove the connector and spray the carb cleaner at the ECM pins as well as connector holes from connecting side. You will see some the carb cleaner will turn into brownish fluid from the other end of connector, respray the cleaner until you see it cleanly coming out of the connector, just spray WD40 on the connector holes and put connector back. If the problem subsides, good, otherwise next time when to go to your mechanic for scanning, ask him to show you individual parameters and you shall note each and every bit. Once note the reading when the engine is working fine and also take the reading while engine is troublesome. Matching both the reading will lead to the findings, and possible solution!
Secondly, get the wiring especially the A/C compressor's checked, as there might be a problem with the A/C clutch being engaged without toggling the A/C switch due to wiring/relay, causing heaviness of the engine.