What was found
AT FL 420, ON PDF 1 ALTITUDE FAIL RED X WAS NOTED, FOLLOWED BY THE WHOLE PDF RED X INDICATION. SHORTLY THEREAFTER, THE CREW NOTED AHRS 1 FAIL, FOLLOWED BY THE ASSOCIATED AUTOPILOT & YAW DAMP FAIL INDICATIONS. RETURNED WHERE MX SWAPPED THE PDF 1 WITH THE MFD & CONDUCTED A CMC DOWNLOAD. NO FAULT CODES WERE NOTED & SYSTEMS WERE OPERATING NORMALLY. ACFT WAS RETURNED TO SERVICE & HAS MADE 2 FLIGHTS WITH NO FAIL CODES.