H- - hoping for some advice/opinion on the following:.

Ford Fiesta Trend 1.1 2019 45,000 miles. We're the 2nd owners. Have had it 18 months. Full service history on it. Used mainly for local journeys 5-15 miles or so.

6 months ago started getting intermittent engine warning light coming on. Took into our local independent garage who diagnosed via error codes faulty O2 sensor, so replaced. A month later, light comes back on. Garage say the new part must have been faulty so replace again. Light is back on within a week. Have spark plugs changed at their suggestion as they're looking dirty and worn. Light is back on and off for a few weeks. Recommended we take to an auto-electrician who sends it for a smoke test at a well-regarded garage who specialise in diagnostics. That comes back all clear so they run a full diagnostics with Ford dial-in team (??). Turns out it's oil contamination due to faulty solenoid. Oil has contaminated the wiring loom and into the oxygen sensor.  Having done some digging it would seem this is a known issue with Fords??  They're quoting nearly £3K to fix - replacing solenoids x2, engine bay wiring loom, lambda sensor and ECU. Does this sound right and if so, are we likely to have this issue again a few 1000 miles more down the road? Obviously no warranty now - is there any point contacting Ford if this is a known issue?? Grateful for any advice/opinions.

There is a similar fault with Mondeo/S-MAX where washer pump seal leaks water into loom which causes corrosion in BCM  which sounds just as unlikely but is real.
SO whilst I know nothing about this specific issue, (where is the info?)  I'd be looking for a guarantee that, if you go ahead with this work, it will fix all faults forever (Ha Ha!)
Changing all that stuff without any inspection (i.e. if ECU isn't showing any signs of contamination, replacement seems excessive?)
What is the oil's route to all these components, how does it get out of the initiating solenoid(s)?
Why can't specific cables be cut and soldered/sealed so that the oil remains trapped?
Dealers are often not "allowed" to do anything other than change bulk components but the choice should be up to you?
As you say 2019 is new (well, I think so) but well outside warranty and whilst likely less than cost to change it is an expensive job and without a Ford contribution/assurance that it won't happen again I'd look for a series of options at different costs that you are "happier" with?
I can only say here questions I'd be asking if I was in that situation...

I would NOT be replacing the whole engine bay wiring loom, just replace the plug and wires to the solenoid.

You will need an independent auto electrician to do that, Ford won't do it because they want to charge you £3k to fit a new harness. 

You've already had two lambda sensors fitted so it's not that

How can they know the ECU is faulty if the wiring is faulty??

I'd test and rectify the wiring first, before I even thought about changing any solenoids, never mind anything else

40 minutes ago, DaveT70 said:

I would NOT be replacing the whole engine bay wiring loom, just replace the plug and wires to the solenoid.

You will need an independent auto electrician to do that, Ford won't do it because they want to charge you £3k to fit a new harness. 

Thanks for your advice.  It's an independent garage that have identified the fault (albeit with the assistance of Ford diagnostics 'dial in') and quoted for the repair. They showed me the oil contamination around the solenoid and where the wiring loom connects. They said because the O2 sensors were initiaiting the fault codes - the oil has reached those too.

45 minutes ago, Shearers said:

There is a similar fault with Mondeo/S-MAX where washer pump seal leaks water into loom which causes corrosion in BCM  which sounds just as unlikely but is real.
SO whilst I know nothing about this specific issue, (where is the info?)  I'd be looking for a guarantee that, if you go ahead with this work, it will fix all faults forever (Ha Ha!)
Changing all that stuff without any inspection (i.e. if ECU isn't showing any signs of contamination, replacement seems excessive?)
What is the oil's route to all these components, how does it get out of the initiating solenoid(s)?
Why can't specific cables be cut and soldered/sealed so that the oil remains trapped?
Dealers are often not "allowed" to do anything other than change bulk components but the choice should be up to you?
As you say 2019 is new (well, I think so) but well outside warranty and whilst likely less than cost to change it is an expensive job and without a Ford contribution/assurance that it won't happen again I'd look for a series of options at different costs that you are "happier" with?
I can only say here questions I'd be asking if I was in that situation...

Thanks for your thoughts.  It feels like a crazy amount of money to spend on a relatively new, well maintained small car. Do you know how you could tell if the ECU is contaminated? They showed me the oil around the solenoids and into the wiring (& that it has reached the O2 sensors as they are what was setting off the engine warning light codes) but I wasn't shown any sign of contamination of the ECU.

The ECU is under the wheelarch (on S-MAX) don't know where it is on yours but it should be in a protected space, perhaps difficult to easily get in to?
If there's no evidence on the plugs/sockets or cables that come from the identified sources then maybe it doesn't need replacing (if it's working, why replace at present but keep a note and pictures of all work done to provide evidence for anything that happens in the future?)
Plus you can usually get ECU repaired rather than replaced at stupid price: https://www.ecutesting.com/
Definitely go with @DaveT70's comments on replacing minimum parts that do the job and isolating any contaminated cables likely with the help of your garage or a good auto electrician?

1 hour ago, Shearers said:

The ECU is under the wheelarch (on S-MAX) don't know where it is on yours but it should be in a protected space, perhaps difficult to easily get in to?
If there's no evidence on the plugs/sockets or cables that come from the identified sources then maybe it doesn't need replacing (if it's working, why replace at present but keep a note and pictures of all work done to provide evidence for anything that happens in the future?)
Plus you can usually get ECU repaired rather than replaced at stupid price: https://www.ecutesting.com/
Definitely go with @DaveT70's comments on replacing minimum parts that do the job and isolating any contaminated cables likely with the help of your garage or a good auto electrician?

OK - great - thanks for your advice. Sounds like more discussion on options is required before going straight in with replacing everything.