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 SHIFT IN DELTA CRUISE AND TAKEOFF MARGIN INDICATIVE OF EGT INDICATION ERRORS. ACFT GROUNDED. REF MM 77-21-01 FOR RESISTANCE CHECKS. FOUND S4 PROBE OUT OF LIMITS. REPLACED S4 EGT PROBE. REF MM 77-21-01. OPS CHECKED GOOD REF SAME.