I took delivery of my 4550 August last year. I noticed that the temp gauge didn't move off the low peg after I after I did several hours of bush hogging and again no movement on another day after an hour or so FEL leaf mulch moving. The dealer took the tractor in during December for some warranty work. I asked about the temp gauge and the mechanic called Mahindra. They told him (as jb1390 posted) that the bleed hose was too large and passing too much coolant around the thermostat. They did a fix and I got the tractor back. During January with the temps in the mid 20s and no snow on the ground, I thought I would go out and play with my new toy and move mulch. I noticed that after an hour the gauge didn't seem to move off the lower end peg. I talked to the mechanic and he said they did run the tractor after they did the fix and the gauge did move for them. Another mechanic suggested that I put a piece of cardboard in front of the radiator to see if that would help for cold weather work. After doing some research I gave it a try. I put a cardboard piece in front of the radiator that left about 5 inches of the top of the radiator exposed. I had another opportunity to move mulch when the temperature was in the mid 20s and the gauge moved to the mid green range fairly quickly. It did the same on several other occasions. I pulled the cardboard now that the temps will make it occasionally into the 40s. However, I have had no opportunity to use the tractor because it is raining every other day and the ground is just too wet.
So what I know from using the cardboard is that the temp gauge works. I hope that, once it gets hot enough, and I use the tractor hard enough, that the temp gauge will move off the lower peg. If it doesn't I will try a smaller piece of cardboard. Depending on the ambient temp conditions there may be another dealer visit for more warranty work. At least I know how to test to see if the temp gauge is working.