Mk6 Fiesta erratic idle and poor throttle response when engine is warm?

Question

I have a Mk6 Fiesta 1.4 petrol that's about 17 years old but has only done roughly 55,000 miles.

It's currently experiencing issues that I'm having a very hard time diagnosing. To summarise the problems with the car - when the engine warms up the idle revs become erratic, there is poor throttle response and the car gets quite juddery when still. However, when cold there are no noticeable issues at all meaning I get about 5 minutes of normal driving before the problems start.

My suspicion is there might be a vacuum hose leak but I've been unable to locate it and I'm also doubting that as it only occurs with a warm engine. The other thing that is confusing me is the lack of any errors codes being produced by the car despite the very apparent issues.

Just to add a bit of context, the car has recently had a new catalytic converter, new spark plugs, a carbon clean, a new front O2 sensor and a full service, but the problems did not immediately follow any of these changes. There was probably 3 months and maybe 500-1000 miles between the last of those changes and me noticing these issues.

Answer 1

So in the end I took the car into a Ford garage for a proper (and expensive) diagnosis. They suggested the front O2 sensor was faulty and upon replacing it, my car is working completely normally again.

On reflection I regret not taking out the O2 sensor and driving without it for a bit to diagnose it myself but for anyone else with a similar problem in the future, that's my advice. It no doubt would have driven better without the O2 sensor as it would have used the default values for generating the fuel/air mix that it uses when the engine is cold as opposed to the incorrect ones being read by the O2 sensor.