just had my head gasket done and my EML light is on with fault code p0420 catalyst efficiency below threshold [bank 1 ] i know it is because coolant was leaking into a cylinder and then burnt off through the exhaust witch the cat did not like and the sensor picked this problem up witch put the EML on . having fixed my head gasket and got the EML turned off it has come on again, problem is would the cat be damaged or is it a faulty sensor, sorry for the long thread