Technical Cat

[SJ2016]

Hi,
I had a new CAT fitted in 2019, but after a couple of weeks the engine light came on so they replaced the part again. Since then the engine light would come on every now again and the garage would switch it off. The garage has now closed (owner retired) and my engine light is coming on more frequently. It passed the MOT ok, but am worried I do need a new Cat, I have had a quote but its £500+ which I can't afford at the moment. My car is a fiat 500 and over 12 years old now. Sorry for long post, thanks in advance for any advice!

 

[portland_bill]

The CAT rarely fails, replacement usually becoming necessary due to impact damage, or rust.

Before the CAT is an oxygen sensor, that measures the exhaust from the engine, and the injection system adjusts the fuelling to keep the readings within a tight margin. After the CAT, is another oxygen sensor, that checks the output from the CAT, to confirm all is operating properly. The sensors work in a harsh environment, and do fail. Or they can last well. (My 2006 Panda is on both originals, the 2017 Doblo just had the 'post-cat' one replaced.)
Expensive CATs might come with new sensors, cheaper ones might not. Included sensors are likely to be cheaper ones. Old ones swapped over may have limited life, but they are usually replaced, because getting old ones out can be difficult.
When the warning light illuminates, it is the computer advisiing that the readings it is getting are outside parameters, or that the internal heating element in one or both sensors has failed. It is important to read the codes, to understand what the computer is upset about. A failing heating element can give intermittent problems, or may only show up after a minimum time, so not an issue on shorter journeys.
Air leaks into the exhaust will give incorrect sensor readings.
Don't guess, don't replace more parts, get the codes read. Once read, don't replace anything, report the results, and let the forum expertise point to the next steps.
(As an example, an incorrect reading from the post-cat sensor could mean, a faulty sensor, a faulty cat, a faulty pre-cat sensor causing incorrect fuelling, or an air leak.) Sadly, many garages attack the effect, not the cause.)
Good sensors come with the correct connector already attached. Cheap sensors supply a separate connector, which can result in poor fitting. Poor wiring will affect the readings. The sensor output varies only by a couple of volts, a poor connection makes a big difference.
Lots to check before spending on more parts.

 

[Derbyfella]

Agree with what’s posted above…

 

[SJ2016]

Thank you for the all the advice!