Mk1 F temp sensor for gauge.

Hi all. Sorry, but I'm beating my head against a brick wall here. My temp gauge isn't working on the dash, and it appears that the sensor has died. (I was getting low readings for a while, then nothing.) My mk1 F has the original black sensor (YCB100370), which you needed the large washer for (MDY100040). The black sensor has been superceded by the blue sensor (YCB100420). It also says that I need to fit the small washer (MDY100080). What I can't see anywhere is whether I fit the small washer instead of the large one, or do I need to fit both? Can anyone please advise?

Best wait until someone more informed about Mk 1 Fs comes along but i could see no harm at all in trying it with both though i would assume it would normally only be fitted with the small washer. The engine block is common to both i believe

This type of sensor is remarkably reliable which is why we still use them & although performance can tail off, they are a thermistor which generally fail “here today gone tomorrow”; that sudden death is mechanical & not electrical.
Do you have a test meter to check its resistance cold & again hot? If you are seeing no difference in reading then it’s a right off & I’ll see if I can make sense of your predicament. I’ve a good sensor in the garage, I’ll check it out tomorrow.
M

And another thought is the sensor body becoming isolated from the block by oxide buildup or more likely the ever so popular sealant liberally applied when a miscreant drip has the temerity to appear.
It doesn’t take much for the sensor/block junction to become high resistance before the gauge starts to drop & pack up all together.
My spare alluded to earlier is a spare due to this very problem & that was one of the first jobs I attended to when I first had the car.
Short the sensor body to the block (jump leads?) & see if normal service is resumed.
M

When using the blue sensor (YCB100420), fit the thinner washer (MDY100080), in place of of the thicker washer.