My engine light on my 2001 GS430 is on. When I went to the mechanic he told me the computer say it is PO420 Catalyst Efficiency Below Threshold bank 1 and 2, and that each convertor would be $1,250 each. My car runs excellent so it was hard for me to believe that it could be bad. Can anyone tell me were to find a cheaper catalyic convertor or can it be the sensors.