Yaris Stuttering/hesitating & engine light on!

[Toby Unna]

Hi all, new Yaris owner - it's a 1.0 on a W-plate.

The engine light came on soon after buying. Plugged the laptop into the OBD port and it says code p0172, mixture too rich. Car runs fine when cold, but after a few minutes' running and from then onwards it stutters/hesitates badly if you push the throttle hard, then picks up after a second and goes ok, though it does feel quite flat (even for a 1-litre!)

Cleared fault codes and turned the engine light off, and it's stayed off for 100 or so miles, but the car is still running poorly.

I removed the AFM and cleaned the hot wire, to no avail.

I've searched around and read that stuttering is a common Yaris issue, but wondered if anyone has any pointers before I start replacing bits hat could make the mixture rich - AFM, coolant/air temp senders, O2 senders?

Maybe worth mentioning that the average MPG readout on the trip computer seems pretty good - ~50mph at a 60mph cruise

Thanks!