As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
#2 ENGINE TREND DATA SHOWS A SUDDEN DECREASE IN OIL TEMPERATURE. GROUNDED - YES. REPLACED OIL TEMP SENSOR AND PERFORMED INSTALLATION TEST, OPS CHECK GOOD IAW B737-800 AMM 79-34-02. DURING INSTALLATION TEST ONLY FAULT MESSAGE SHOWN WAS 78-21492.