It wouldn't be constant if it was map, a friends map sensor went at the weekend, we ha diagnostics on and pointed to that, I actually thought it was lambda at first since it sounded like a subaru lol
I think in all honesty diagnostics should be the first thing, even if the light isn't on the Ecu can still store fault codes etc. So for the £40 quid you will know whats right and wrong, rather than spend slot of time effort and money going on a sensor change mission, because it could something as simple as the engine temp sensor, or it could be icv
Also you should really get diagnostics done and this problem sorted before you go for a remap cause 8/10 id say the mapper will just map over the fault and it will still be stored on the ecu, and the issue still hidden there. I know others will disagree but I seen it this weekend, a mate had a lot of fault codes and couple of running issues with a cammed 16v and he had a remap an it ran worse than a non mapped engine, with the faults still there
Like I said diagnostics is your best option here rather than ponder over it for ages and replace all the sensors mentioned on here and to then find it could be something very simple, it may cost £40 and upwards on diagnostics as they will charge an hourly rate even if they spend 15mins on the car, but it's better than spending £150 on sensors and still have the problem