C3 Catalytic converter P0420 fault

[bobins]

VF7**************[VIN obfuscated, can be read by forum staff]
C3 2013 1.2 EB2 about 65k miles.
Regularly serviced.

More of a sanity check - but brother in law's C3 has put up a P0420 Catalytic Converter - Loss of Efficiency fault. Cleared the fault and it's stayed off for now.

So the order of things to look at.....?

Lambda sensor wirings
Cat physical condition (leaks etc)
Sensor outputs via the OBD reader (if I can find where they are !)

Then if all else fails - replace one or both sensors, followed reluctantly by the cat ?

Any other places to look at ?

[PaulC5]

If you look at the live data on your code reader, the inlet lambda sensor should be constantly switching but the outlet one should be fairly static. If the outlet one is also constantly switching it suggests the catalyst needs replacing since it is not doing much. Do the checks once the engine has warmed up.

[bobins]

Ta for that, Paul.

[ozvtr]

If you look at the live data on your code reader, the inlet lambda sensor should be constantly switching but the outlet one should be fairly static. If the outlet one is also constantly switching it suggests the catalyst needs replacing since it is not doing much. Do the checks once the engine has warmed up.

The engine may be running a wide band sensor in which case the output of the upstream sensor (bank1 sensor1) will have a varying output voltage but it wont "swing" or "switch" much at all. Count the wires to the sensor. 4 wires, narrow band. 5 wires wide band.
However the downstream sensor (bank1 sensor2) will be a narrow band sensor. This should remain relatively static. Swinging or switching of the output indicates a problem.
The wide band sensor should have a small oscillation around 2.5V. An upstream narrow band sensor will swing around a center point of about 0.5V (0.1 to 0.9V).
The downstream sensor should remain (nearly) static at about the 0.5V mark.
Typical damage to the catalytic converter is caused by oil consumption.
Leaks in the exhaust manifold can set off a catalyst error.