Don't know about the sensor, but may be a good idea to test it, use a thermometer and a pot of boiling water. Not bleeding the cooling system properly (fill it, run engine until hot and the thermostat opens, fill it more - and use the proper BLEND of antifreeze/distilled water).
You should be able to clear the codes even with a cheapo OBD2 reader, just clear 'em and see if they come back (often a local car parts store like Autozone can do this cheap or free).
I think you have a big old air bubble in your cooling system, the symptom you describe of the temp gauge being dead cold then suddenly going to full hot is a perfect example of this. Hope you didn't damage the head gasket when it overheated. The same thing can cause the cooling fan not to work.
Interesting.... Be sure to post the results. As I overheated my r53 when my T-Stat stuck closed. I mean you could have fried eggs on the engine. Cracked the Head also. Anyways I went to the 180 * t-stat, new head and gaskets.. and I too am getting the "below normal operating temp". I didnt change the sensor when I changed the head. My data logs show no issues with temp. SOOoooo... be sure to post your results.
I have been running the code ( clear... then returns) for a month now. As far as a cold engine ( cooler engine) it might bother the ECU, and maybe puff out a bit more emissions... but it is better to run on!! ( cooler is denser fuel).
So I installed the correct thermostat and new temperature sensor. I ran the engine until the temp gauge got to just below normal (I have a manual cooling fan) then the needle got to normal and then dropped down just below normal (thermostat kicking in) and there she stayed with no fan. So I reved to 3000 for about 5 mins with the temp needle just below normal. I checked the engine and noticed the expansion tank was now filling up with boiling coolant even though the temp needle still remained below normal. The WRX radiator was hot but not nothing excessive. I thinking now the pressure cap is defective allowing coolant to escape at a low temperature thus giving a low temperature reading? Next I will replace the cap
So I'm back in the US and just fitted a new pressure cap( old one looked bad) the temp gauge went up a little closer to normal but still when the engine is running on idle(no fan) the temp gauge stays at normal but with the new pressure cap I can see fluid starting to fill up the explansion tank which must mean the engine is getting hot. I don't see how this can be a an airlock so maybe the very large Subaru radiator is affecting the cooling somehow. I'm going to block off half the radiator airflow with cardboard to see what happens.
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