My daughter has a 2013 Ford Focus 1.0 Ecoboost. She bought this early on in 2024. We are having the quite common problem of the Engine Management Light(EML) coming on with the code PO420. To cut a long story short it does the usual. I reset it and it comes on again after a few weeks. I have changed both pre & post lamda sensors and it still does the same. I have read on other threads the following information:-

Possible P0420 causes:
-damaged or failed downstream(rear) O2 sensor
-downstream O2 sensor wiring damaged or bad connector/plug
-engine coolant temperature sensor is not working properly
-damaged or leaking exhaust manifold, catalytic converter, or cat pipe
-failed or underperforming catalytic converter
-bad fuel quality
-oil contamination to cat

Reading other posts, it seems that I should just byte the bullet and replace the catalytic converter?

The car has just passed its MOT test (I fixed a front coil spring) with no issues with the smoke test.

Does this indicate that the catalytic converter is really ok and that I should consider the other options...maybe the sensor wiring damaged or bad connector/plug?

The car is running well, with no obvious issues. The only other issue I am aware of, is that there is a slow coolant water leak. I am 99.9% sure this is coming from the small expansion tank, which I intend to replace in the next few weeks. Part of the problem is that the car lives over 50 miles away, which makes it hard to gain access to it. I will change this and reset the light.

I am lucky in that the car passed its MOT. My daughter uses the car every week.

If I installed Forscan would this help me further to diagnose the problem or should I just wait for the weather to improve (next summer maybe) and just change the catalytic converter then?

I am a DIY novice and would really appreciate any comments from anyone that has experienced of this problem or knows how best to solve it.

Thanks for reading this.

This fault usually is cured by a new cat on the 1.0EB.  Though personally I wouldn't replace it until there are more symptoms.

Petrol's don't have a smoke test at MOT.  Their test is for Carbon Monoxide, Hydrocarbons, and Lambda (air/fuel ratio).

The fact that it passed the Lambda test suggests there isn't an issue with fuelling or with the lambda sensors.  And the fact that it passed the CO test means the cat must be working to some extent.

The problem here seems to be that the cars ECU test is much tighter than MOT emissions test.  Also the MOT is generally carried out with a hot cat in a stationary position.  So, for example, while gently cruising on a flat motorway the cat will cool down and won't work as efficiently.  The second lambda sensor will then detect excess oxygen in the post-cat exhaust, which it determines to mean the CO hasn't been converted into CO2 as it should've been.  There are no CO sensors in the exhaust so it can't test for that directly.

1 hour ago, philp1963 said:

My daughter has a 2013 Ford Focus 1.0 Ecoboost. She bought this early on in 2024.

Unrelated to your current problem, but have the wetbelts been changed ? If not they are over due.

I am so grateful to people that take the time and effort to respond on this forum.  Not being a mechanic, it is difficult to know if what I am doing is the best way forward, and it is so helpful to get other peoples input.

 TomsFocus: Thank You.  The information you gave about the MOT test was very very helpful to me.  Due to the car not being easily available to me, I think my plan will be to wait for the summer and then arrange with my daughter to have the car for a week and replace the cat.  It means that I do not have to panic if the cat fails on the MOT test.  I just need now to look into where best to buy the replacement cat. 

Unofix:- Thank You.  The timing belt was changed not long before we bought the car, about 5000 miles ago(Jan 24).  The car has done about 120000 miles. Its had a really weird history.  For some reason according to the service record it had a new engine and gearbox fitted in 2019 at 65570 miles.  I was told that it had been a ford recall (Done at a HENDY garage). I could ring the garage up I suppose to try and find out more, but the engine does have a sticker on it stating where it was made and the date.  Whats weird, is that why would you fit a new engine without using a new cam belt?  However, when we bought the car, I got the impression from the seller that the wrong oil had been used in the car and that it had affected the cam belt and thats why the cam belt was changed. I was bogged down with work at the time, in hindsight I should have got this information in more detail.  But the car seems to run well, other then the EML, so I dont think I need to worry too much about its history? 

 

Thanks again.

 

I said in my last post on this thread that I would wait until the summer and replace the cat in warmer weather. 

However something very odd has happened.  My daughter has been driving around with the EML(Engine management light) lit up.  I have not bothered resetting it (no point, and car is too far away from me to bother).  For no apparent reason the EML has gone out on its own accord????  When you turn the ignition on the EML comes on briefly...so the light still works.

I did'nt think that the EMU reset error codes...I assumed that you need to manually reset it yourself. 

Can anybody explain this?

Perhaps it might be a wiring problem after-all???  Although when I reset it originally, it defiantly had the PO420 code registered.

Any thoughts?

Thank you again.

The ECU continues to do self-testing for many codes, so they can clear on their own if conditions change.  There are some codes that have to be manually cleared.

Thanks Tom.  I did not know that.  I really thought that all codes had to be manually reset. I guess I should have googled that.

I guess I will now wait to see if the light re-appears again.  Is it possible that me changing the air filter(which was quite dirty), the spark plugs and the post-lamda sensor has helped fix it, even though it did come on again after I had done these changes. Maybe the CAT has had time to clear itself.... I know I am probably thinking garbage...very wishful thinking on my part. 

I shall have to wait and see.

Thanks