As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DIAGNOSTICS REPORTS NR 1 ENGINE TREND DATA SHOWS A SUDDEN DELTA EGT INCREASE AND EGTHDM DECREASE. EGT INDICATION ERRORS ARE SUSPECT. GROUNDED - YES. FOUND S4 RESISTANCE READING HIGH AT 309 OHMS. S1 READING 1.0 OHM. PRECAUTIONARY REMOVED AND INSTALLED NEW S4 PROBE REF AMM 77-21-01. AFTER CLEANING AND S4 REPLACEMENT READINGS. S1=43.46, S2= 21.10, S3=23.16, S4=52.36. PERFORMED NR 1 ENGINE RUN. FAULT CODE 77-20871 DOES NOT SHOW ACTIVE CURRENTLY.