what is the difference of the two temperature sensors (one in on the engine and one on the bottom of the radiator)? which one controls the fan? and what does the other do?

 

Both sensors feed directly into the ECU, the primary is used for ECU adjustments to engine/drivetrain components and the secondary to manage the fans [along with a pair of relays] w/ and without AC running, at least that's my understanding.

 

Here is a link to help explain engine temp units. http://www.autoshop101.com/forms/h32.pdf

 

thanks!!!

wy does the USDM Fit have a secondary temperature sensor at the bottom or the radiator?

 

The secondary controls the AC Condenser Fan via the ECU and a pair of relays.