B752 reported by ATC as being 400 ft low, crew switched xponders. Readout, on altitude.

Attributes
ACN	496450
Time
Date	200012
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : zab.artcc
State Reference	NM
Altitude	msl bound lower : 32600 msl bound upper : 33000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zab.artcc tower : dtw.tower
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 8000 flight time type : 280
ASRS Report	496450
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : undershoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	flight crew : returned to assigned altitude other
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Facility Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

ZAB called and asked for our altitude. We advised FL330. They advised we were 400 ft low. We switched xponders and asked for a new altitude readout. They advised FL330. In a later sector we swapped xponders back and had no deviation.

Original NASA ASRS Text

Title: B752 RPTED BY ATC AS BEING 400 FT LOW, CREW SWITCHED XPONDERS. READOUT, ON ALT.

Narrative: ZAB CALLED AND ASKED FOR OUR ALT. WE ADVISED FL330. THEY ADVISED WE WERE 400 FT LOW. WE SWITCHED XPONDERS AND ASKED FOR A NEW ALT READOUT. THEY ADVISED FL330. IN A LATER SECTOR WE SWAPPED XPONDERS BACK AND HAD NO DEV.

Data retrieved from NASA's ASRS site as of July 2007 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.