Narrative:

Our clearance was to squawk code XXXX and at the gate code XXXX was entered into the transponder. After receiving initial taxi instructions; we were advised by ground to ensure we were squawking XXXX. I looked down and confirmed the code in the transponder was set properly and responded to ATC that sometimes our transponder fails to squawk properly on the ground but should work once airborne. (Note: this is a recurring issue where ATC does not see our code when by all accounts it should be able too.) as the first officer; I switched over to clearance delivery to receive a minor reroute while the captain continued to monitor ATC ground during the taxi out. After receiving the reroute we were advised to monitor tower which we did and then received clearance to takeoff on runway 30 and fly heading 250 and then contact potomac departure 125.05. Again; no mention of our transponder was made on the ground except by the ground controller after initial taxi instructions. After check in with potomac departure we were flying our assigned heading and given clearance to climb to 7;000 ft. ATC also queried us to squawk XXXX and identify as they said they were showing a squawk of 1200. At this time I told the controller we were indeed squawking XXXX and it was here (after being told by potomac that they were not receiving XXXX) that I switched from transponder 1 to transponder 2; hoping this may correct the problem. Potomac cleared us direct blues on course; but continued to tell us they were not getting our transponder code and instead were receiving 1200. We advised that we were squawking the correct code and continued to fly the clearance we had last been given. Potomac then gave us an 'off vector' of 30 degrees or so away from blues which we followed. Shortly thereafter we were re-cleared to blues and I asked the controller at this point if he was getting the XXXX code from us and he advised us that he was receiving it correctly. He then switched us over to the next controller and advised us to copy down a telephone number for a possible 'pilot deviation' for squawking 1200. We copied down the number and switched frequencies. At no time from leaving the gate until landing did the transponder code windows show anything other than our assigned code of XXXX. Remainder of the flight was uneventful.

Related reports:

Google
 

Original NASA ASRS Text

Title: An air carrier crew reported that their aircraft's transponder showed a 1200 code even though the proper ATC assigned code was displayed in the window.

Narrative: Our clearance was to squawk code XXXX and at the gate code XXXX was entered into the transponder. After receiving initial taxi instructions; we were advised by Ground to ensure we were squawking XXXX. I looked down and confirmed the code in the transponder was set properly and responded to ATC that sometimes our transponder fails to squawk properly on the ground but should work once airborne. (Note: this is a recurring issue where ATC does not see our code when by all accounts it should be able too.) As the First Officer; I switched over to Clearance Delivery to receive a minor reroute while the Captain continued to monitor ATC Ground during the taxi out. After receiving the reroute we were advised to monitor Tower which we did and then received clearance to takeoff on Runway 30 and fly heading 250 and then contact Potomac Departure 125.05. Again; no mention of our transponder was made on the ground except by the Ground Controller after initial taxi instructions. After check in with Potomac Departure we were flying our assigned heading and given clearance to climb to 7;000 FT. ATC also queried us to squawk XXXX and IDENT as they said they were showing a squawk of 1200. At this time I told the Controller we were indeed squawking XXXX and it was here (after being told by Potomac that they were not receiving XXXX) that I switched from transponder 1 to transponder 2; hoping this may correct the problem. Potomac cleared us direct BLUES on course; but continued to tell us they were not getting our transponder code and instead were receiving 1200. We advised that we were squawking the correct code and continued to fly the clearance we had last been given. Potomac then gave us an 'off vector' of 30 degrees or so away from BLUES which we followed. Shortly thereafter we were re-cleared to BLUES and I asked the Controller at this point if he was getting the XXXX code from us and he advised us that he was receiving it correctly. He then switched us over to the next controller and advised us to copy down a telephone number for a possible 'pilot deviation' for squawking 1200. We copied down the number and switched frequencies. At no time from leaving the gate until landing did the transponder code windows show anything other than our assigned code of XXXX. Remainder of the flight was uneventful.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.