Dave is correct - lock code 188 for high-temp alarm. Not sure why your display is not going to the lock settings, though... Try 188 and see what happens.
Sue, I recall your problem being the high alarm setting. That is factory set at 290 degrees for AH1, the box probe. AH2 is set at 190 degrees for the meat probe. The code to change the AH1 and AH2 setting is 188, not 155.