Hi, don't want to highjack the thread but I'm in the middle of something similar.
My local mechanic says it is trial and error and can be expensive if you buy new parts. My problem was with the cold start and intermittant engine symbol appearing. Car wouldn't tick over when cold. Once warmed up went perfectly other than random speedo readings.
Got a throttle body (£30) O2 sensor £30 and temp sensor £20 off local scrapper. Some error codes disappeared but the cold start persists. The mechanic didn't change the speedo sensor in the gearbox as he said it looked welded in.
Both scrap throttle bodies we tried caused crazy idling even when hot - so we've gone back to the original. Scrapper says they just bolt on but mech thinks they might need coding in (he hasn't got the codes)
New shape Fiesta Spring 2004 (dealer demonstrator) I'm back to square one. On warm mornings, no problem. Cold mornings, five minutes with foot on throttle until its warmed up. Mechanic says there is a hidden breather pipe behind the throttle body with a pinhole that causes it to collapse under acceleration. Possibly sucking in air, he's sealed it with silicone as it is a dealer part. No change.
Trial and error is OK but not at £35 per hour........