Sensor error on IMAC

The MAC "hangs" on grey screen on startup. Progress bar stops at approx. 1/4 along. Apple logo is present. After a few minutes the mac reboots, this process repeats, hang, reboot etc. After running MAC Hardware test from the internet I get error code: 4SNS/1/40000000:TH00-9.000 which says sensor error. Anybody know what sensor and am I looking at an expensive repair (out of warranty of course). I tried all resets etc with an apple support person. Nothing worked and they did not know what the sensor was!!!