GA pilot is advised by ATC that the mode C altitude is different than the altitude on the altimeter.

Attributes
ACN	446150
Time
Date	199908
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	navaid : pae.vor
State Reference	WA
Altitude	msl single value : 11000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zse.artcc
Operator	general aviation : personal
Make Model Name	Cessna 340/340A
Operating Under FAR Part	Part 91
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	other
Function	flight crew : single pilot
Qualification	pilot : multi engine pilot : cfi pilot : atp
Experience	flight time last 90 days : 250 flight time total : 4000 flight time type : 300
ASRS Report	446150
Person 2
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	atc equipment other atc equipment : radar other controllera
Resolutory Action	controller : issued advisory none taken : anomaly accepted
Supplementary
Problem Areas	Flight Crew Human Performance Aircraft
Primary Problem	Aircraft

Narrative:

Controller said my transponder/mode C indicated 11700 ft while I was positively level at 11000 ft with current sea/bfi altimeter (actually, both before and after I was given the current setting, my altitude was accurate).

Original NASA ASRS Text

Title: GA PLT IS ADVISED BY ATC THAT THE MODE C ALT IS DIFFERENT THAN THE ALT ON THE ALTIMETER.

Narrative: CTLR SAID MY XPONDER/MODE C INDICATED 11700 FT WHILE I WAS POSITIVELY LEVEL AT 11000 FT WITH CURRENT SEA/BFI ALTIMETER (ACTUALLY, BOTH BEFORE AND AFTER I WAS GIVEN THE CURRENT SETTING, MY ALT WAS ACCURATE).

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.