Ford Fiesta 08 1.6 duratec... High idling

Hi All,

Please can someone help....
Car was involved in front left impact , new wing , bonnet etc ...it's now Idling at about 1100 rpm... Engine management light on ... Error code p0131 ... Oxygen sensor .... They removed cooling system to repair damage ..... Noticed that the coolant has dropped down to below minimum ... No evidence of any leaks .... If you turn on air con, revs dropped down to about 800 rpm , and the fan stays on constantly ... Ford fiesta 1600 duratec engine .... Could it be faulty coolant temperature sensor...
The Car has been back to the garage , twice ... Each time sent back saying everything ok...
The car was working perfectly before incident ..
The body shop say it's ok to drive, but it's running rich

Thanks in advance

Stephen