Rough idle and limp mode with useless fault code

I have a manual 1.5 TDCi Ford Fiesta Titanium from 2018 with 120ps and 113500 miles on the clock. The car has trouble with limp mode, which clears on stop/start button press cycles. The error code has only ever complained about the airmass sensor, even one’s newly installed.

Before replacing the airmass sensor:

·         The car would go into high idle after starting almost every time

·         Would go into limp mode when 3500rpm is exceeded

I could just about live with this. However, it seemed this masked the following issues that arose once a new one was installed:

·         Rough idle, especially when the AC drives (Feels like a misfire and will stall on cold start if AC is on)

·         Limp mode now appearing randomly (Even after cruising or decelerating, to be discovered next time I press the accelerator, which is also sometimes the cause)

I installed a new EGR valve which made the car more responsive, but also much slower before it boosts, where it is as fast as expected out of limp mode (~1800rpm, perhaps because the EGR is now also open until then? This could just be the nature of the car…) but didn’t fix the above issues. The mechanic ran some tests on the fuel injectors and said that their readings are acceptable, however if that is speaking relative to each other, then all of them could be equally bad – I am unsure of the control variable he used.

Other preventative maintenance I’ve done in the last 7000 miles includes:

·         Full new set of glow plugs

·         New timing belt

·         Injector cleaner in current tank of fuel

·         Full service

The car passed its MOT with no advisories last month and averages 65mpg. I would prefer to try and fix it than trade it in. It makes sense to tackle the parts that are most likely to be the cause, given they’ve covered so much distance, so any suggestions for likely culprits would be greatly appreciated.