AN A320 FLT CREW HAD AN AIR DATA COMPUTER FAILURE WITHOUT ANY FAULT INDICATIONS DISPLAYED. DISCOVERED ON TAXI OUT WHEN PIC NOTICED A PLUS 9 DEGS C DIFFERENCE BETWEEN THE CURRENT ATIS TEMPERATURE AND THE OAT DISPLAYED BY THE ACFT.

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DURING TAXI TO RWY 1R; I NOTICED TAT AND SAT TEMPERATURES WERE AT -01 CELSIUS. THIS DIFFERED FROM ATC ATIS OF +9 CELSIUS. I HAVE FOUND THESE SENSORS TO BE QUITE RELIABLE AND FELT FURTHER INVESTIGATION WARRANTED. NOTIFIED MAINT AND WAS UNABLE TO ASCERTAIN WHY TEMPS WERE DIFFERENT. NO ECAM OR SD INFO WAS SHOWING ANY FAULTS. WENT BACK TO GATE; AND FOUND AIR DATA COMPUTER #2 WAS FAULTED OR FAILED. NO ACFT WARNING OCCURRED OR SHOWED A FAILURE OF AIR DATA COMPUTER #2. WHEN FO ON 3 SELECTOR SHOWED APPROPRIATE TEMPS; CAPT ON 3 AND NORMAL SHOWED ERRONEOUS VALUE. WHEN ADR DEF AND SELECTED OFF; FO ON 3 OK; BUT CAPT ON 3 AND NORMAL SHOWED AMBER XX'S. I WRITE THIS TO ALERT THE AIRBUS DEPT THAT AN ADR FAILURE OF SOME SORT OCCURRED BUT WE RECEIVED NO WARNING OR ALERT. TEMPS; ETC; WERE ALL GREEN AND ONLY MY NOTICE OF DIFFERENT VALUE OF TEMPERATURE INDICATED ANY PROB. THEY SWITCHED ACFT ON US AT THE LAST MINUTE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE RPT WAS WRITTEN UP MAINLY TO ALERT AIRBUS THAT A FAULT EXISTED AND THERE WAS NO WARNING. THESE TEMPS WERE ALL IN THE GREEN AND DID NOT READ THE SAME AS THE FO'S; WHICH WERE READING NORMAL. THE ERROR WAS +9 DEGREES CELSIUS.