What was found
FLIGHT COMPARTMENT PACK HOT CAUTION LIGHT ON. COMPLIED WITH CHECKLIST. AFTER PACK TURNED OFF A/C WOULD NOT MAINTAIN PRESSURIZATION. PERFORMED EMERGENCY DESCENT. LANDING UNEVENTFUL. REMOVED AIRCRAFT FROM SERVICE. INTERROGATED CDS. BASED ON FOUND CODES & HEX CODES. R & R THE COCKPIT BYPASS VALVE IAW AMM 21-51-31-000-801, & 21-51-31-400-801. OPS TESTED GOOD. PRESSURIZED ACFT IAW AMM 21-00-00-863-801 & 21-00-00-863-802. TESTED GOOD. FAULT CODE 4201 (ECU DIG CH 1) PERSISTED. PAGE 7 OF CDS ECS HEX CODES (ECS FLIGHT DATA R CH) SHOW ALL ZEROES. R & R THE ECU IAW AMM 21-61-01-000/400-801. FAULT CODE 4201 CLEARED. NO OTHER FAULTS. OPS CHECKED GOOD. RETURNED TO SERVICE.