Breezy_Pete

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Rad fan testing by linking radiator thermoswitch connector:

Have you still got the sensor that you pulled out? Had same error on our Fabia very recently, seemed to give OK temp numbers but kept logging that fault, even put the EML on while missus was out and about. Replacement fixed it, and only thing apparent when examining old one was that the junction of brass to grey plastic had gone a bit loose; could twist the two bits relative to each other through a small angle with little effort.

Others that share that splice are G42/G71 IAT/MAP sensor and G40 Cam position sensor.

White wire from pin 1 at sensor connector goes to ECU connector pin 74, brown earth goes via a splice called 'sender earth' in the engine loom to pin 54.

Furnish me with engine code or VIN and I can give you pin numbers at engine ECU tomorrow.

As you have figured out, the temperature sensor is driven/read by the engine ECU, and the results communicated to the cluster only via CAN messages. If I were trying to pin this down I think I would log (with VCDS) coolant temp as reported by engine ECU during some driving, and look for any glitches in the data; or generally implausible numbers. I wouldn't, in fact I think that might be my prime suspect. Do you have pinout info? Any moisture in the connector at engine end? Engine ECU fault really unlikely I think.

Your front brake PR code is 1LG, 340mm discs; rear is 1KT, the 272mm solid 10mm thick discs.

@RossMacGill Chuck me your VIN by private message, I can look up.

Indicated 90 covers a pretty big span of reality, so yes, still possible I would think.

I'm seeing two 12V feeds on pins 18 and 20 of a 20-way connector at passenger front window motor, red/yellow thin one and red/white thick one, respectively. Got both? Measure relative to earth on pin 19 to test earth return intactness.

No, it never got anywhere near the top of my to-do list.

Maybe previous owner took thermostat out?

Ping me your VIN by private message and I'll try to find.

There's only two buttons on the scan tool. 😁 Total milage since car was built; only done about 15-20k since transplant, at a guess.

😊 Thanks for asking. 'New' engine is serving us very well. Done a couple of little jobs on it very recently, Coolant temperature sensor (2004 vintage) started throwing fault codes and a warning light last week - proud of the missus for scanning it while out and about with handheld tool that lives in the glovebox. That got swapped out for another I had sitting around. Faults gone. While doing that I noticed that the little heatshield on the exhaust manifold was hanging on by one screw, so I bunged a replacement on yesterday. MOT a month back mentioned (nothing but) deterioration of the fuel hose that I'd replaced (with new, generic) at the time of the swap. Tester said he himself was struggling to find aftermarket stuff that didn't crack, and to let him know if I found anything decent! I decided to grab a genuine replacement from Skoda, hopefully that won't go the same way. About another 5k til the car passes 200k miles, I think. Oh, and a nice man in a big van came and took away all my scrap metal for me on Saturday, including the old engine, after I'd stripped it of all the possibly useful bits. 😁

I find it always takes a journey or two, as opposed to just letting it sit idling, before the coolant level settles down after any such change. Make sure it's got enough antifreeze in, fair chance a freeze-up was what killed the old waterpump I should think. Probably why the previous owner got rid?

Well done, glad you found the problem. I thought that lack of return flow in the little hose at top of expansion tank was pointing in that direction.

Measure mV across ends of each of the strip and blade fuse in engine bay with everything off and closed except bonnet. That should start to narrow it down.

I'd expect a trickle from that hose, increasing a little when engine revved.

Please let me know where you find output tests for the rad fan(s), I just failed to do so.

Sounding much more like a simple coolant temperature sensor/connection/wiring fault now. When I read the bit about the cabin heating going cold at the same time as the very high indicated temps, I couldn't imagine what could be going on with the coolant (which flows through the matrix all the time, regardless of settings). Then it occurred to me that there might be a safety interlock which prevents cabin air being directed through the heater matrix if the coolant temperature is being measured at e.g. >120C. No idea if that really is 'a thing', but it could be.

If the A/C isn't working well enough to trigger any such activity, this image should help you to test instead by linking contacts of the radiator thermoswitch connector (daylight job, and not that easy to access, approx 100mm above radiator bottom hose connection):

If it has A/C that does work it gives a quick way of checking the radiator fan(s). Within a minute or so of switching on A/C the fan should start up at low speed (relatively quiet). If instead it does nothing for a longer time then comes on at full speed (loud!), you probably have a failed low speed function on the fan. It's triggered via the refrigerant pressure when A/C is on, so no need to have hot engine to try this.

I would investigate the radiator fan function first. Does the car have A/C, and if so does it work?