Ford Focus

1.6 MK2 06

155k Miles

My cars Engine light has just lit up and im getting a P0420 code Cat threshold error when i have plugged my reader in. the light came on a year or so ago but then cleared itself after a few weeks

what do i need to check on the 02 sensors, the voltage ?

how much am i looking at for a new Cat assuming it needs replacing?

On a side issue when my car is idle seems to shudder, pulse eg when sat at lights etc like its going to stall but doesnt which i can feel through the car.

could be a number of issues, but normally it will indicate a problem with the catalytic or the post cat sensor

i would start by clearing the code and see if it returns

if it does, then would check the operation of the post cat sensor...tbh, although you can carry out voltage checks on the sensor, you'd be far better off and safer, viewing it in live data...voltage and resistence checks can actually damage the sensor.

because of the code, its unlikely to be a wiring/connection problem (though possible) anyway.

Have u checked pcv pipe

Hi yeah cleared the code but it had returned.

Where would I find the Pcv pipe?

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Underneath inlet manifold t piece rubber hose rips or collapses in on itself

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ok take a look

pluged the fault code reader in and the secondary 02 sensor voltage is moving up and down and is not stable. the first is moving up and down also.

freeze frame data says short term fuel trim is 0.78 on bank 1 and long term fuel trim on bank 2 0,78

so your sensors are switching, but you need to take note of the voltages they are switching to/from...sensors can switch but stay rich or lean...you need to know if they switch from rich to lean

also need to note an average of fuel trims rather than a freeze frame...bearing in mind that if you had just cleared the code, those trims will settle to a base value, so therefore, when codes get cleared, ideally you will need to take for a run for at least 10 mins before getting an accurate trim reading

also need to confirm what type of figure the reader is giving you

that is...if it reads percentages, (base value 0%) then 0.75% is showing a good burn condition

but if it reads a base lamda value (which will be 1) then 0.75 is showing a very rich condition