Hi. So the EML light on my 2018 1.0 (100ps) Fiesta keeps coming on. Say’s emissions are out and the Cat is inefficient. I’m told the Cats hardly ever go wrong and it should be one of the 2 Lambda sensors. However I’ve changed them both now and the light is still coming on. I’ve also been told the spark plugs might be the cause but they were changed less than 12mths ago. 
 

The lights been coming in now for about a year and is getting more frequent.  I just keep putting it out with my OBD reader. About 6mths ago the wet belt went and I had to have a part engine rebuild. However the light was coming on before this. Could it still have something to do with it?

Best to post the Code you are getting and if you are using a generic code reader then there may be Ford specific codes that you are not seeing.

Cats do wear out on these.  How many miles has it done?

Hi. It’s done 74k. Everyone tells me the Cat should be ok with this mileage. 

On 8/15/2023 at 2:59 PM, Tizer said:

Best to post the Code you are getting and if you are using a generic code reader then there may be Ford specific codes that you are not seeing.

Hi. It’s a P420 code. I know that’s quite a generic one tho. 

Same problem with mine 2016 1,0 Ford Focus Ecoboost. I also think it is the cat! The check engine light /Р0420/ come on first at 82 K km. Interesting fact is that I have the light on every time I drive on the motorway at high speed. With city driving I still have no warning light. Tried several cat cleaner fuel additives with no significant effect. After the first additive I have no problems for about 5 K km, but now it start again. I don't know if true but I have red that driving the car with cat not working properly may cause heat cracks in the cylinder head.

On 8/17/2023 at 8:05 AM, Chris70 said:

Hi. It’s done 74k. Everyone tells me the Cat should be ok with this mileage. 

Ah, they can't be all be wrong then!  

Are those suggestions based on experience with the 1.0 EcoBoost engine or just cars in general?  The 1.0EB cats are more prone to failure than most modern cats.  At 74k I wouldn't be at all surprised if it was the cat failing in this case.

2 hours ago, Tony Roman said:

I don't know if true but I have red that driving the car with cat not working properly may cause heat cracks in the cylinder head.

Or maybe cracks in the Cylinder Head cause the P0420 Codes or otherwise damage the Cat because the Fuelling is mucked up, who knows.