Fault code P0420

The MIL light came on, so I used a scan tool for the above code. The can runs fine, but I would like to know what this explanation Catalyst Efficiency below Threshold (bank) 1 actually means.

The car is a C3 sx 1.4 petrol 52 plate with 50.000m

The reader is AUTEL can obd11 Maxiscan MS300

I have erased the code & the mil light is out, but this is the second time it has happened within a month.

Any advice would be welcome.



Citroen C3 fault code reader

I would like to know what this explanation Catalyst Efficiency below Threshold (bank) 1 actually means.

It means the car has detected (or thinks) there is a problem with the catalytic converter.

This could be caused by a failing catalytic converter, faulty O2 sensor, leaking exhaust system or some other fault.

Thanx Arfur for info. I will have a visual look asap, but if nothing obvious is it true if the CAT rattles its faulty?

If the 02 sensors need replacing, can they be undone without a specialised socket tool?

I will have a visual look asap, but if nothing obvious is it true if the CAT rattles its faulty?

Have a look at this typical catalytic converter
Its made up of a block of substrate that is full of holes along its length.

You get a rattle from the cat when this substrate gets damaged and breaks up, it the bits of substrate that are actually rattling around inside. This means the cat is not going to work so well, or work at all because its the substrate and its structure that does the work.

Roland wrote: If the 02 sensors need replacing, can they be undone without a specialised socket tool?

C3 Haynes manual (here on amazon) may help.

You can pick up a new O2 (Lambda) rear sensor for Citroen C3 2002 1.4 here

And the Catalytic Converter for the C3 (here from euro car parts), but its expensive to change bits without being sure they are actually faulty

Thanx Arfur for reply. I have made a quick inspection of exhaust system & nothing seems wrong. The Cat appears quiet. I suspect the oxygen sensor if anything, so have ordered a proper 22mm tool for removal. Can I check it in any way?

Roland wrote:Can I check it in any way?

A garage would stick an oscilloscope on it and look at the signal and compare it with a reference signal from a known working system.

DIYers are usually limited to a visual inspection. Look at the electrical connections, then with the sensor removed look at the sensor head for damage.

https://www.hella.com/techworld/us/Tech ... nsor-4379/ this link has O2 sensor tests and faulting information.

Web Archive of that page is here.



The site could always do with some photos if you manage to get your O2 sensor off

Thanx again, will wait now until the tool arrives so I can check properly.

Today I have removed the Oxygen sensor from the front exhaust manifold. The wiring & sensor all appear in good condition. The picture shows a 4wire type with a sooty coloured end, which I presume is okay?
The fault code reappears about every 5 days but the car runs fine.

Roland wrote:The picture shows a 4wire type with a sooty coloured end, which I presume is okay?

It looks ok, but you can't always tell by looking. Its not showing any classic signs of failure, but that's not to say its failed or has an intermittent fault.

Did you try a new sensor in there?

Thanx Arfur for input, but since putting it back NO MIL as yet. Its been over a week now, & car is fine. However im off to Oulton park on Mon so we will see after that. Will keep you posted.