We've had our 2013 XJ 3.0 for about 4 months now and the Low Coolant warning came on the other day for the wife. Checked it and it looked like it was at the line but I put a bit in it to top it off. Sure enough the next day after driving a bit the warning came one again. The wife says it will stay on until she shuts the car off and then it will come on after driving a bit. Looked at it again and it's still up at the same level. I am guessing that it just the sensor but anyone have any thoughts or experiences with this?

 

Same exact situation about a year ago. Time for a new sensor.

 

kccode1, when you top off the tank, are you using water or are you putting in 50/50 mix coolant. Something I ran into awhile back when doing my own coolant change was that the sensor needs to see a mix of coolant and water to work correctly. If you put in too much water (ie, dilute down the coolant to something less than 50/50), the sensor will be covered, but will tell the computer that the fluid level is low. If you have been adding water for awhile to make up for the minor losses, you may want to consider adding some coolant concentrate and see if the light stays out that way.

But, like what drpainless said, if you have been adding 50/50 mix coolant, then your sensor is toast.

 

I topped it with the 50/50 premix but I only had to put about 1/2 cup in anyway. I did see the issue with the float

 

I have a 2011 XJ supersport today the coolant level low warning came on. This was following an emergency stop. I cycled the ignition several times after but the problem has remained.
Having read multiple forum posts on the subject suggesting replacing the tank and sensor I figured I would try the simple stuff first.
Given that the coolant level has not changed in 3 years of owning the car I figured it had to be the sensor. The car does always put out steam from the exhaust when the weather is cool and when examining the steam with my finger and tasting (sounds gross) it is just water vapour and not the dreaded head gasket leak. Water is a bi product of petrol burn so I expect it in a 5Litre engine.
As I did a rapid stop I figured the coolant had shifted to the front of the tank and set off the sensor. Maybe as I stopped on a rough country road maybe it was jolted and got stuck.
I waited for the coolant to be cool enough to remove the tank cap and inspected it. I considered poking a pipe in the an blowing some air in but lack of a pipe I resorted to giving it a bit of a Clarkson. I started shaking the tank and gently banging it and also gave the connector a wiggle and checked it was connected properly. After a minute or two of tapping the tank and shaking I put the lid on. I turned on the ignition and the warning light had gone.
I would suggest giving it a good tap and inspection and maybe having a gentle probe if this fails before it becomes and expensive fix.
I did note the coolant does smell a little hot or burnt but it's colour and condition coupled with the way my car runs would suggest this is probably normal after 25k miles. It tasted as it should and will get a flush as dictated in the service manual.
I know my problem was just caused through an extreme driving situation but it should still be considered if the problem arises. I would also take note that when freeing the stuck float the coolant was close to operating temp so may of made it easier to free off.

 

This post is almost a year old. I tested the sensor by lifting it manually in the tank. It was a bad sensor. I found a brand new factory tank with sensor and put it on. Still cost me about $100 but I got a new tank with it so all is good. Sensor seems to be a week part in these cars.

 

The low coolant warning on my 2013 XJ came on about 2 months ago. My first thought was a bad sensor, and so i replaced the tank, but the warning remained on. I did a host of troubleshooting. I verified that the connection to the computer was working properly by using a small gauge wire to short the the signal (wire inserted into the sensor female plug pin holes); results: shorting = no warning message, no shorting, i.e. open= yes warning message. I then checked the sensor. I knew that the sensor operated using a float which had an 'open circuit' when the float was down (i.e. 'dry condition'), and closed when the float was raised ('high liquid level'). I knew this by two methods: 1) i cut the old sensor out of the tank and tested it with a ohm meter 2) I tested the new tank's sensor using the same ohm-meter and using a stiff wire inserted through the fill hole to move the sensor up and down and confirming a open-when-down, and closed circuit-when-up.

Everything checked out electronically, so i tested the theory that somehow the coolant had gotten watered down and caused the float to remain permanently down (very skeptical of this). But, using the sensor i had removed, i tested it in two solutions, one water and one fresh 50/50 coolant. And indeed, the float did not rise in water, but did in the correct 50/50 coolant - yikes! But how did my coolant change? I speculate that when i took it into a quickie oil change place two months ago, they must have done me the favor of topping off the coolant with water - HA!

Also, i include a pic of the the sensor i cut from the original tank for inquiring minds.

 

An old thread, I know, but it helped me with my 2011 Jaguar XJ Supercharged "low coolant level" warning problem, so I'll add my experience:
I started getting an intermittent "low coolant level" warning a month ago. I checked the level - it was fine, so I just ignored it.
The warning became consistent so I eventually got around to checking it out. With the car cold, I removed the coolant fill cap and shone a flashlight through the tank and looked down into the fill orifice. I could see the black top of the sensor (see jwbenn's photo in this thread). I could also make out the off-white float beneath this black top. I fashioned a hook from a piece of light copper wire and fished around for the bottom of the float. I was able to "pull up" on the bottom of the float and make the float rise. Since the float was fully submerged in coolant, this verified that the float was no longer buoyant.
I suppose the correct thing to do at this point would be to order a replacement used reservoir (including sensor) on eBay and do the job properly. Nope. I decided to give my float a life saver! I made a lasso with that fine copper wire and fed it under the float. I pulled it tight and confirmed it lifted the sensor float. I then fashioned a float from a Styrofoam egg carton and small zip-tie. I attached the fine copper wire to my new floatation device and worked the floatation device under the surface of the coolant (with a pair of long tweezers).
With the assistance of my Styrofoam life saver, the sensor float is sufficiently buoyant to stay up when the coolant tank is full. I confirmed the float still falls if the coolant level goes below my life saver.
Warning light is off. All is well.