does the knock sensor have to be replace or could it be something else?

Hello, The knock sensor issue will be separate (most likely) from the "gas cap" code. The knock sensor may even just have a dirty or loose connection, or wiring may be faulty/left unplugged. The fan should only run when the relay is on, so the last fan may have been fine. Check the fan by jumping (shorting) across the relay with a 10 gauge or larger wire. If the fan comes on, you know the relay needs to be tested. It may be difficult, because it seems that the issue with the fan may be intermittent. The "Gas Cap" code informs you that pressure in the EVAP system and fuel tank is being lost to some sort of leak. This leak may be a crack, hole, or something missing, or it may be a bad purge valve (allowing the engine to suck air from the fuel tank at all times). This is the vacuum leak side of the issue that must be investigated.