Hi, I'm having an ongoing problem with my 2002 2.0 Mondeo lx and it's frustrating me no end. Basically the engine management light came on a while ago so I plugged in OBD (on board diagnostic) machine and it showed up with the code P0420 which it says is a fault at the catalytic converter system/bank 1 and the effeciency is below the threshold. I have changed both the lambda sensors but the problem still exists and the same code comes up.

Could there be a problem in the wiring or somewhere in the system?

Please help! Many thanks in advance.

if you can try testing the sensor for output by connecting to it a digital voltmeter, set to a 2v scale. Connect the +ve test lead to the sensor signal wire [see wire colours] and the -ve test lead to earth/battery -ve. The voltage produced should swing from app 0.6v to 1.0v and back around once per second. If the voltage is stuck high then the engine is probably running rich for some other reason. If stuck low then usually either a faulty Lambda Sensor,or an air leak.