Relationship between downstream 02 sensor and Catalytic Converter.

[Joe144]

Hi folks hope you are all well, 

                                                Just trying to investigate a P0420 DTC. Question at the moment is does a downstream 02 sensor malfunction

on account of a bad Cat. Convertor or can an 02 sensor go bad of its own accord, wear and tear ?  S1 seems to be within the paraments .1 v - .75v

oscillating more or less between those two figures. Reading for second 02 sensor seems askew, oscillating  almost the same as  S1.  Apparently S2 is

suppose to maintain a steadier reading eg.  .75 -.78 or a little above.  What would the erratic reading on S2 point too do you think ? A bad Converter , a bad

Sensor or perhaps both ? Any thoughts ? 

 

[stantheman1]

It's very likely to be a bad cat. but a bad downstream o2 sensor can't be ruled out completely. If you have access to an infrared thermometer then check the temperature of the fully warmed up cat. the outlet should be hotter than the inlet.  

[Joe144]

 

 

 

1 hour ago, stantheman1 said:

It's very likely to be a bad cat. but a bad downstream o2 sensor can't be ruled out completely. If you have access to an infrared thermometer then check the temperature of the fully warmed up cat. the outlet should be hotter than the inlet.  

Thanks very much Steven, do you know what the gap in temperature should be between the outlet and inlet ? 

[stantheman1]

It should be around 100 degrees C hotter. Once the engine has reached normal temperature raise the rpm to approx. 2500 before taking the measurement. 

Please let us know how you get on.