Thermostat open circuit F265 Code 17700

[Henry9090]

Car is a A4 B7 1.6 Petrol ALZ Engine
I have a permanent error code 17700 - which says the thermostat is open circuit.
I have replaced the thermostat and it made no difference (except for my bank balance)

I was thinking it could be the wiring, but the thermostat appears to work well - the temperature of the coolant is rock solid at 90 C and never budges after it warms up.

I saw somewhere that there is a fuse in the thermostat circuit. Does anyone know where this fuse might be?

Might it be a fault in the ECU where it senses that there is an open circuit, but there isn't?

It's really annoying - not only do I have a permanent ECU error light, but the ECU thinks there is a cooling fault so turns on the radiator fan, even these mornings when I'm scraping ice off the window!

Thoughts welcome. Thanks.

[Henry9090]

Just an update on this one.

I sent the ECU off to a guy who repairs them and he has confirmed it is the ECU at fault; he has reproduced the problem on the bench. However, he says he can't fix it because it is the printed circuit board itself that is faulty as opposed to any of the components. I'm struggling to believe this at the moment - if it's the PCB then it can usually be fixed with a wire jumper.

That leaves me trying to find a replacement ECU. This 1.6 litre petrol model wasn't sold in the UK but was in Ireland because of the tax laws being anti-CC at the time (now anti-CO2) and so it could be difficult to find a used ECU.

Does anyone know where to find the schematic diagrams for Siemens ECUs? I reckon if I had the schematic I could maybe fix it.

[Henry9090]

Another update....

I have been focused on the purported "Faulty ECU" and even went to the bother of ordering chips from China. Having now replaced the chip that drives the thermostat the problem is still there. So, this got me thinking that maybe the ECU is Ok after all. I have now tested the F265 thermostat circuit from the ECU pins and it is indeed open circuit, as the ECU has been reporting from the beginning. It is supposed to be 14 to 16 ohms.

So much for the ECU test guy....

So, it is either the wiring between the ECU and the thermostat or the thermostat itself is open circuit. I suspect the latter.

However - hang on, you might say, haven't you already replaced the thermostat? Well, no I didn't. A mechanic is supposed to have changed it but now I am wondering did he do it at all. It is very, very awkward to get at and it is possible that he simply didn't bother!

For now, I'll give him the benefit of the doubt and say it is the wiring. As I said, it's very hard to get at to test, but hope to get to the bottom of this sometime soon.

Also, I removed the fuse for the radiator fan so that it isn't running permanently any more. There is certainly no need for it at this time of year. (ECU is complaining about that too though, as you might expect, but the light is on anyway so its no more annoyance to me).

I'll send another update if I get to the bottom of it. Alternatively, there are some nice used cars coming into the dealerships at this time of year and my patience is wearing thin....

[Henry9090]

Well I did get to the bottom of it.

I got another mechanic to replace the thermostat and guess what - all OK!

He gave me the one he took out and it is obviously not new. The car was first registered in January 2005 and the old thermostat has a date code of 2004.

So the guy that said he replaced it evidently didn't and wasted a LOT of my time. Everything from sending the ECU to be diagnosed (by another guy who didn't know what he was doing) to sending off to China for replacement chips. The ECU told me exactly what was wrong in the first place - the thermostat was open circuit. Yes, it was open circuit when checked on a multi-meter.

I thought it was likely when the Audi spares guy said "we hold loads of those in stock as they go all the time".

In my earlier years I'd have replaced it myself and saved a lot of bother. It seems it's only when you do it yourself do you know what's been done.