likely the connector for the sensor which is just inside the front sheet metal on the inside of the engine compartment is corroded. Trace the sensor to the connector, disonnect the two halves and give it a thorough cleaning, spray with contact cleaner, let it dry and reconnect. Good chance it will restore accurate temp reading function to your OBC. Resistance goes to infinity with a corroded connector which causes default of minimum temp.
HTH,
George