2002 Nissan Pathfinder • 203,000 miles

My car started idling high at startup and then staying very high even after a few minutes. My mechanic replaced the IACV and a sensor and it ran fine for about a week and half and then reverted back to idling high and revving. He tried for a week to reset the computer code or reset the computer but it worked successfully then went back to erractic. I finally took it to the dealership and they toid me that the mechanic should have replaced two sensors when he replaced the valve and now the ECm is ruined as a result. They want about $2100-$2200 to fix it. A few months ago I spent $2500 on various repairs and now, recently laid off from my job, I am not sure if I can spend this. Is it true that this problem could have happened as a result of not changing one of the sensors? Do I have any options?
January 15, 2013.

Honestly, I never heard of that. What two sensors are they talking about?