Ok. The whole sorry saga.
Coolant leaked, tracked up a cable as far as the engine ECU. The pins and sockets of the ECU and the loom were totally corroded and the plastic had melted. The main dealer was correct . The ECU was scrap as was the loom, hence a 5 hour labour charge to repair and replace as the loom stretches as far as the headlights. A total Audi price of over 2k obviously prompted me to seek an alternative.
Most other garages and Auto elecs I contacted though, simply did not want to know.
Eventually I found an Auto electrician who was willing, and more importantly able, to clone and re-code an ECU .
https://theautoelectrician.co.uk/
I sourced probably the only salvaged ECU (every model derivative is different) in the country from a company in London called Nirro Ltd.
Despite mixed reviews on Google they did me proud, £198 including next day delivery.
My auto elec sourced for me probably the only loom in the country too.
The loom was spliced in, the ECU cloned and re-coded, the leaking part changed and the car is now warning light-free.
Total bill including parts and a shed load of labour £970.
See attached picture of the offending leaking valve that caused the whole issue. Picture 2 shows the totally corroded pins.
I have pictures of the melted ECU and loom on Whatsapp but can't upload them here.
Did I notice the coolant level dropping?
No I didn't. The only time I venture under the bonnet is to top up the windscreen washer fluid and the coolant level was fine the last time.
You don't even need to lift the bonnet to check the oil any more, there's no dip stick and it's checked on the MMI screen.
I had the car serviced at a main dealer only about 6 weeks ago and if the coolant level was low then they didn't see fit to mention it.
I can only assume that the leak was so small as to be barely noticeable but, in such a place that it tracked directly into a cable and up into the loom.
Everyone who has seen the car is in agreement that they have never seen damage so extensive caused in such a way.
Having shelled out nearly £900 this time last year for a new water pump and run-on pump I've totally lost faith in the car.
7 years old , 60,000 miles and... FSH ferchrissakes...! It's going and I'm looking at a new non-Audi replacement.
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