A B747-400 was grounded after landing because Maintenance determined that the maintenance action to comply with MEL 36-21-3 was not properly performed prior to the flight.

Narrative

Prior to our preflight at our originating station we reviewed that the aircraft log book had a current airworthiness signature for our date of flight and departure. We also noticed that the Flight Release paper work had an MEL 36-21-3 for the #2 Engine Bleed Pressure Sensor which was also reflected in the log book. Upon referencing the MEL we noticed that Operations Procedures were NONE and we agreed that the proper placard for this MEL was located approximately 2 inches to the left of the Hydraulic Pressure Gauge and slightly below the Captain's Primary Flight Display. 36-21-3 ENGINE BLEED PRESSURE SENSOR. REMARKS AND EXCEPTIONS May be inoperative. OPERATIONS PROCEDURES NONE. OPERATIONAL LIMITATIONS NONE. PLACARD Placard below Captain's PFD on P-1-1 panel: '_____ ENG BLEED PRESS SENSOR INOP.' Enter item in Aircraft Deferred Item Log. MAINTENANCE PROCEDURES NONE. Immediately after engine shutdown Maintenance personnel advised us that our aircraft was grounded (AOG) due to an improper repair of MEL 36-21-3. Prior to flight we followed the proper procedures for determining airworthiness of our aircraft and were under the impression throughout the entire flight that we had an airworthy aircraft. Flight continuation was canceled.