Land Rover Repair: Land Rover mech. question., coolant temperature sensor, p0420 catalyst system efficiency below threshold bank 1

Question
Hello,

Thank you for your help regarding my questions with code "P1138".. It really helped! I replaced all of my O2 sensors. Now the code is cleared.

Another code has come up--
"P0420 Catalyst System Efficiency Below Threshold (Bank 1)"
Do you have any tips for fixing this problem?

Looking forward to your reply.
Thank you,
Silvano

Answer
Hi Silvana,

This problem is on the downstream side of the catalytic convertor.  There are several possible causes:

Leaded fuel was used;
Oxygen sensor is faulty;
Engine coolant temperature sensor is faulty;
Crack in the exhaust manifold, catalytic converter or somewhere on the exhaust pipe between engine and sensor;
Spark timing was retarded.

You should measure the voltage of the oxygen sensor on Bank 1 (the sensor after the converter). The voltage should be low and steady. If the voltage varies too much (e.g. 100mV to 600mV) then you should consider replacing the catalytic converter.  I hope you have separate cats.  My older Disco has a Y pipe and both cats would have to be replaced.

Best of luck,

JohnMc