Backstory: In April my car suddenly had an EPC light and the coolant temperature quickly began to rise during a short test drive. It reached about 110°.
We replaced a couple of temperature sensors (one on the radiator outlet and the other on the right hand side of the block), that didn't make a difference and we eventually diagnosed it to a faulty thermostat and replaced the entire housing (brand new, OEM from ebay).
That fixed it! All was well, EPC light had gone, coolant never passed 90°.
Fast forward to last week, just set off from work, in stop-start traffic when I got a "Oil sensor: workshop!" error message on the dash. Odd. I rolled forward as the traffic moved and the message disappeared, came to a stop again and it came up reappeared (Stop start was enabled on the car when this occured). I switched the MFD to oil temperature and I joined the motorway (in eco mode and keeping the revs as low as possible), the oil temperature was slowing climbing and kinda balanced out at 111° but during the drive home, coolant sat at a constant 90°.
I have never seen the oil go above 105, even in my old 1.6 Mk5 golf so I was a little worried. I checked the oil level when I got home and it was within the marks on the dipstick, so it wasn't low.
Out of curiosity (maybe in hindsight, stupidity), I continued to run it. The oil usually sat around 111 on 20 minute drives and once maxed out around 114-115. The same "Oil sensor: Workshop" kept popping up the when stop-start engaged. Until one day, when it popped up as I was driving down a hill doing about 50mph and the temperature on the MFD was jumping from 83 to between 50 and "---" (it's hard to explain it's almost like the sensor was restarting and just pulling a value from nowhere, I have a video of similar behavior in the imgur link below).
Definitely looks like a sensor problem right? So I called my local VW dealer for a price on a new sensor, I though the oil temperature sensor was part of the level sensor (which is located in the sump), according to the sensor that's already in there (P/N 03F907660D), it's not.
Problem 1, I don't know where the oil temperature sensor is (is it the one that we replaced on the right side of the block?).
What's more interesting is, I recently ran it up to temperature, again oil climbed to around 110 but this time, the coolant didn't reach 90° on the gauge. It reached 70-80° and it started fluctuating, dropping about 5° before slowly climbing again. We left it idling and realized the coolant temperature was eventually dropping and reached about 62° before we shut it off. The coolant tank was hot enough to be well over what the gauge was telling me and had plenty of fluid it in it.
What would cause the coolant temperature to drop (according to the dashboard gauge) but the oil temperature to rise?
My only guess would be that it's the thermostat again but any suggestions or any steps to further diagnose this would be very much appreciated.
https://imgur.com/a/sNsB5aV