Last fall I noticed that my temp display was not registering anything even after an extended period of time on the trail.
No symptoms of over heating but decided to proceed with due diligence anyway. I pulled out the coolant temperature sensor and proceeded to check it out by heating water in a pot and checking resistance as the temperature increased. It behaved as per the diagnostic procedure in the service manual. Next, I checked continuity from the sensor right up to the display and all appeared OK as well. I chalked it up to a defective display and not about to change that expensive part. No biggie I said to myself. I can ride without it.

In December even though it was -10C after about an hour of technical slow riding the MIL light came on solid. No problems detected and it cleared itself once I parked overnight. It was fine in the morning. I did take notice at that time that the fan did not seem to come on or certainly not as frequently.

Fast forward to today. A balmy +2C so decided to look at the issues a little deeper. I brought up the temperature of the coolant to 90C as measured with an external precision thermometer and the fan never came on. I jumped the themo switch wires and the fan kicked in without any issues.

So, long story short I just placed an order with Alibaba for the thermo switch and the coolant sensor just for the heck of it.
Total price including shipping $49 CAN.

UFORCE 2016

[2015 Z Force 800 53/Trail EPS]

Hope that fixes it

Last fall I noticed that my temp display was not registering anything even after an extended period of time on the trail.
No symptoms of over heating but decided to proceed with due diligence anyway. I pulled out the coolant temperature sensor and proceeded to check it out by heating water in a pot and checking resistance as the temperature increased. It behaved as per the diagnostic procedure in the service manual. Next, I checked continuity from the sensor right up to the display and all appeared OK as well. I chalked it up to a defective display and not about to change that expensive part. No biggie I said to myself. I can ride without it.

In December even though it was -10C after about an hour of technical slow riding the MIL light came on solid. No problems detected and it cleared itself once I parked overnight. It was fine in the morning. I did take notice at that time that the fan did not seem to come on or certainly not as frequently.

Fast forward to today. A balmy +2C so decided to look at the issues a little deeper. I brought up the temperature of the coolant to 90C as measured with an external precision thermometer and the fan never came on. I jumped the themo switch wires and the fan kicked in without any issues.

So, long story short I just placed an order with Alibaba for the thermo switch and the coolant sensor just for the heck of it.
Total price including shipping $49 CAN.

UFORCE 2016

Is your temperature gauge is reading one bar? If so then you probably lost ground on the temperature sensor. The engine is path to ground. My gauge stayed on one bar and I went through the testing and two sensors. One day I just randomly checked continuity between the thermostat housing and the cylinder head and I had nothing. I tagged a wire across the two surfaces and I had continuity! I guess corrosion had built up on the two mating surfaces. Remove the thermostat housing, use a green scouring pad to clean the mating surfaces between the housing and the cylinder head, reassemble and with the original sensor reinstalled my gauge began reading again.

That is an excellent tip.
Yes, the gauge stays at one bar and last fall for maybe half an hour it worked again and then stopped.
I did temporarily ground the engine with a booster cable right to the battery terminal but that never helped any.
What you are saying makes a lot of sense and when I receive the new sensor I will follow your procedure and clean up that area. Probably will check with my OHM meter first. It is a real pain to get into with having to remove the plastic panels and seat.

I will post my results in about a month once I receive the parts from overseas and tackle this little project.

Edit: Looking at the manual and thermostat housing assembly I may simply add a 14 gauge wire under one of the cover bolts to a solid ground.
In my opinion that would prevent future corrosion problems that may re-occur between the 2 mated surfaces.
I will look at that option as well. Lots of vibration though so not sure how well that would hold.

That is an excellent tip.
Yes, the gauge stays at one bar and last fall for maybe half an hour it worked again and then stopped.
I did temporarily ground the engine with a booster cable right to the battery terminal but that never helped any.
What you are saying makes a lot of sense and when I receive the new sensor I will follow your procedure and clean up that area. Probably will check with my OHM meter first. It is a real pain to get into with having to remove the plastic panels and seat.

I will post my results in about a month once I receive the parts from overseas and tackle this little project.

Edit: Looking at the manual and thermostat housing assembly I may simply add a 14 gauge wire under one of the cover bolts to a solid ground.
In my opinion that would prevent future corrosion problems that may re-occur between the 2 mated surfaces.
I will look at that option as well. Lots of vibration though so not sure how well that would hold.

Hope you efforts are successful!

Allright! Fast forward 3 months later today and finally got the parts direct from China.
The economy is slowly starting to roll again after this lock down with this new normal.
I was running my UTV with a shorted relay bypass so the fan was always on to prevent overheating.
It worked OK but was annoying.

Anyways, I proceeded to remove the PITA covers to get at the engine. What a chore that is including having to remove the drivers seat for better access. I started with what I thought would be the easy part which was replacing the radiator mounted thermo switch for the fan.

Nope! After I did the swap I suddenly realized that the connection was different.
Snip the wires and made a match.
Tested and works fine. The fan starts cycling at 80C

Next, my temp gauge was not working so tore into that looking at the ground side of the temp sensor.
Sure enough I was measuring 1.1K ohms. Way too much of a resistance to make that sensor work properly.

Took the housing apart and found corrosion on the bolts and the mating surface.
Shined everything up with emery paper and the gauge started working again.

Since this is a low mileage machine I decided to improve on the grounding aspect of that particular sensor.
I do not wish to have another repeat and feel this is a problem area and poor design.

So, utilizing a 1 inch gear clamp and a piece of 12 gauge wire I provided a secondary ground to the thermostat housing.
The engine ground bolt is conveniently located only 4 inches away so tied the other end right there.
I put a light coating of dielectric grease after I was done to repel water and resist corrosion.
I left a generous loop to absorb engine vibrations and all is good again.
At least now if the fan thermo switch fails I will get advance warning.

Hate to drag a thread back from the dead, but this one pretty much covers all the bases.
This sounds like me.   My fan comes on when it should, that's fine but have the default 1 bar on the gauge.
Something I haven't seen mentioned is hard starting, cold.

My Z500 lives at my cabin.  It sits for weeks in the Canadian winter, I reach in, turn the key and it starts right up.  But last time I was at the cabin, it started hard.  Had to get in, and feed it about half throttle to get it running...then keep on the pedal to keep it running for about 30 seconds, then it was fine.  Didnt ride it enough to notice if the was working correctly.
Was there this weekend.  Started hard again. This time it sat idling while I did other stuff, heard the fan come, then off, a couple times. Looked and noticed the 1 bar on the temp gauge..odd.
Thinking about it on the way home....I figured there must be a fan switch on the rad, which is working fine ...but the sender for the gauge (and I'd presume,the ECU) would be a 2nd unit.   I figured that one was likely the culprit, since it's starting hard and no gauge.  But reading some treads, especially this one, makes me think it's worth adding a supplemental ground and then trying a cold start. 
If it's a ground issue like it sounds, adding a ground will likely resolve both my issues.
Since I won't be back there for at least a week, might as well.post and ask....
Anyone with the 'one bar on gauge' problem also experience hard starting that also went away when you resolved the ground issue?
And again, Canadian winter, so when I say cold start, it's well below freezing.

[2015 Z Force 800 53/Trail EPS]

What year z500 is this? The older ones have a fan switch on the radiator but I think newer then 2018 don't. Both however use a temp sensor on the engine for ECU and dash gauge needs. The one-bar thing may be that sensor failing as many have...or a failing dash pod. Hard starting may be low system voltage while cranking or low fuel pressure. I hope you are keeping the battery on a tender while not in use and in the cold, If not, it may now be due for one...and a tender.

Mine's a 2020.  Canadian model, which may or may not make a difference...  (Remember that fuel primer knob on my Canadian BF 650i and everyone said I was nuts?  haha)
Lots of battery....it'll crank for days and not even slow down.  And, last time to the camp I pulled the battery, brought it home, and put it on my maintainer because I was thinking I wasn't going to be using it for a couple months.  Was on for a couple weeks. 
I want to say it's really acting like it's unsure of the outside temp, so it's not richening it enough.   And we're talking 5*f or so. The previous winters, same storage, same everything, I could reach in, prime the pump, and it was running within a second or 2 of starting cranking...and it would idle nice and smooth.     
Now, it won't start cold unless I give it half throttle or more, and it'll start and I have to keep my foot into it for at least 30 seconds...if I let off it just stalls. After 30 or so seconds of warming up, I can take my foot off the gas and it's running fine...and that's the end of the drama until I cold start it the next morning.

It's not 'choking' it enough to start, and it's not choking it enough to idle in the first 30 seconds...but after getting a little warmth in it, it's good.   And once it's good (after that 30 seconds and idling unassisted) I can shut it off and it starts right up with nothing but a bump of the key. 

Both issues (the lack of temp gauge and the cold start) popped up at the same time...I've only ran it twice this winter...and both issues were there both times.
I'd think that one or both of those would trip a check engine light, but they're not.

Oh, and the fan is coming on, running, and turning off exactly the way it should.   

So....on the drive home yesterday I figured that it must be the same temp sender for the ECM and the gauge, and there would be a on/off temp switch in the rad.  I'm just knocking stuff around in my head for now.  And of course, it's at the cabin so all I can do it sit home and think until at least next weekend.

<Edit>   I think I found out the scoop on the 2 'sensors' vs 1.   Going by the parts lookups, my 2020 lists the water temp sensor (ECM and gauge) and also the thermoswitch (on/off for the fan)....but for the 2021 and 2022 I only find the temperature sensor and no thermoswitch.    So I'd venture a guess that my 2020 was the last year for the 2 sensor set-up, and the 21's and 22's started using the temperature sending unit for both purposes.

I think I'll try an auxiliary sensor ground, then a cold start.  If it starts good, then I'll run a permanent ground wire.