Fan running full speed

Dear All,

In my system Dell Precision Tower 3420 both fans, Processor Fan and System Fan are running at full speed. It is discomforting and in short run it will accumulate lot of dust in the system because of increased airflow.

Fans themselves are OK. I have tried disconnecting system fan then Processor fan still runs very high. System is neat and clean. Physically system is not heating. Operational environment is not hot, just normal.

I have tried reverting BIOS configurations to Factory default, system default etc. and tried configuring other settings. I have also upgraded BIOS to latest available on Dell but no avail at all.

Dell BIOS diagnostic utility ePSA shows

Alert "Warning one or more thermal sensors were out of range"

HWinfo64 shows ambient temp sensor issues. There is also an option to manually set fan speed at owner's risk in HWinfo64. I tired to slow down fans manually in it and then fans become silent and rpm becomes low as should be in normal functioning.

System is not physically hot but fans running at full speed. If it is some in-built sensor defect, it may slowly halt the system at some point making it dysfunctional.  

any pointers ?