I also have a 2017 Cadillac XT5. I was driving it the other day and noticed the temperature gauge never moved. It stayed on C the entire time, I drove for over two hours straight. Also, the fan runs constantly and I had the "steering assist reduced" message. The car ran great the whole way and I never lost power and the engine didn't seem to get hot. Would it be that the thermostat is bad or it needs a new coolant temperature sensor or both?

 

A tech needs to determine that. Most common failure is the T-stat but occasional/rare sensor failures too. Both components are in the same area, might not be a bad idea to do both at the same time if there is some gray area in the diagnosis.

 

I have the same problem. Guess I'll go check the 12V system.

Be sure to check the negative battery terminal connection as I have read that there was a problem with the hardware that made it so the connection could not be made reliably tight.

 

It is not surprising that the battery starts to fail after a while with Auto Stop Start technology. Has any run into these issues with the system completely disabled (i.e. using Smart Stop-Start)? Technically, you should not even require an AGM battery if you can make sure the voltage is consistent.

 

The charging system is designed with AGM in mind.

 

For anyone interested I too had the dreaded “action required steering assist is reduced " on the center display of this 2017 XT5 with 37k miles on it ...It had the same symptoms as some previous posters had mentioned ...Radiator fan running at its highest speed... Air conditioner stopped working... power steering seemed to be laboring or nonexistent and the temperature gauge needle showed no temperature...it read no temp the entire time the car was running. I tested the car with a code reader and it showed P0128

I decided to buy the coolant temperature sensor and the thermostat .. The plan was to replace the coolant temp sensor and see if that was all it needed. Best laid plans. Welp, I experienced so much difficulty in getting the sensor out all the way I surrendered and drove it to a shop…The shop used my parts as well as an intake manifold gasket, new antifreeze and charged me labor. Finished in a matter of hours with everything restored back to normal. The tech who did the work told me it was the thermostat.

 

Hello Jim
It's ironic you took a picture of your dash. I have a 2017 XT5 and every so often the "steering assist is reduced" flashes up on my dasdboard. Believe it or not I have one tire that has a slow leak and when I added air to 36 PSI, the message goes away! I can't see it being the cause, but with computers who knows! I see your pic has one tire low on air. I do not have any other issues happening with the assist message. Sounds crazy I know, but my message goes away! - Ray

 

Just had the same thing happen at 47.3k miles on my 2018 XT5 Platinum. Steering assist message, no A/C, engine temp not moving, and idling at 1k RPMs. Thanks to everyone on this thread I figured it was either the battery or the thermostat. Bought a battery at Autozone for $200 (ouch) and put it in myself. Lost Tune Select saves on SiriusXM and had to reset the HUD info. But it seemed like everything else survived the loss of power. Seat presets, Carplay, and Bluetooth all still were there.

Overall, it was pretty painless to change the battery and was able to do it myself in about 25 minutes. Best part about it? It fixed the issue. Immediately on starting it, no Steering assist message, idle down around 600 RPMs, and A/C was blowing cold. Thanks to everyone who contributed on this thread. I wouldn't have had any clue without reading this thread and several others.

 

engine temp not moving

Where is your engine temperature after the battery change? Leaning toward what @Cutlass says.

 

It’s definitely not the battery. That’s a coincidence. It’ll be back and you’ll need a new thermostat…

 

Agree. The battery wasn't causing the issue here.

 

As a follow-up, headed out yesterday for a 300 mile round trip. Steering assist message came back along with same other symptoms at 47.9k miles. Scheduling service for next week. I am going to disconnect and reconnect the battery to see if it will reset it again as I have some driving to do this week.

 

Gotta get that thermostat swapped out. That’s the solution that eventually has proved itself as the fix, plus there is a service bulletin for it.

 

I wasn’t aware there was a TSB for this specific issue. Do you know the TSB number?

 

Curious if the failing thermostats are caused by low coolant level? Can anyone verify that the coolant level was indeed within the hashmarks on the side of the coolant surge tank at all times before the thermostat failure?

 

Curious if the failing thermostats are caused by low coolant level? Can anyone verify that the coolant level was indeed within the hashmarks on the side of the coolant surge tank at all times before the thermostat failure?
View attachment 609088

This is an interesting question. My 2018 XT5 was low on coolant from the factory when I bought it. I filled it to the top of the hash marked area as shown in the about picture. No problems thus far after three years and 23,000 miles. Seems doubtful that this could cause a thermostat to fail, but stranger things have happened.

 

 

Ignore all the dust. Just got back from West Texas.

 

That is right where the level should be when the engine is cold. At operating temperature, the level will rise toward the top of the hash marks.

 

So, it does not seem related to coolant level then? Just confirming. Not sure what the dealership will say, but have an appointment at the dealership on Saturday morning. Gathering information to give them. Apparently the thermostat issues are a GM thing and not just the XT5. There are some long threads on the '17 GMC Arcadia having the same exact symptoms and fix.

 

Well, in an interesting turn, all the symptoms just went away this afternoon. I had to drive 30 minutes away for an errand. On the drive there I had the Steering Assist message, no A/C, and pinned coolant temp gauge. Got to the place and turned off my car. When I turned it back on I was surprised to feel cold air. Had 2 more stops on the way home and the symptoms stayed gone. Still have my appointment at the dealership on Saturday. I was hoping that the symptoms would be present when I took it in. Probably doesn't matter since they will pull codes anyway and do work based on that.

Any suggestions on keywords or TSBs I can use to make sure they replace the thermostat?

 

Coolant level is a separate issue from the thermostat issue.
The thermostat trouble code is for the coolant temp BELOW normal operating temperature. If the coolant level was actually really low, the coolant temp would be hot…too high.
Low coolant level on an XT5 or XT6 is almost always from a leaking radiator. Leaky radiators are very common on these 2 Cadillacs.

 

I have periodically had this message pop up on my message center dashboard. I think that it is related to weather/rain/carwash ( I think I had seen this on the forum last year).
I also occasionally have the check engine light come on also. Example: last Tuesday the Steering Assist message came on, then after several stops and starts the check engine light came on and both stayed until Friday. Then they both disappeared(turned off). Saturday evening (yes, it rained) the Steering Assist Reduced message returned. I have an appointment scheduled for Monday, but would like to go to it armed with more knowledge than I currently have. All information would be appreciated. Last January I had several messages constantly popping up on the dashboard and the dealer did a software update which seemed to help.

I had a similar issue with my 2017 XT5. During my research in this forum, I identified a possible Software update (or lack of a Software Update) as the culprit. I was at the dealer when I read the description of the issue and the solution. I immediately contacted the Service Manager and requested that he check the Software. The Service Manager had informed me that there was no issue with my vehicle, but the service agent did not check the vehicle software for updates. I needed two updates. After the updates, the issue did not appear again.
I would also suggest that you check your battery as well. Based on the Autozone battery test, do not assume that the battery is good. The Autozone test indicated that the battery was slightly below the optimum voltage but was still good. At the same time, that may apply to a lead-acid battery in another vehicle. A decrease in the AGM battery used in the XT5 causes the monitoring electronics to issue a warning. The warning, which did not reference the battery, appeared and disappeared regularly. After replacing the battery, I never got the warning message again.

 

I keep my coolant surge tank, when cold, at the bottom of the hash mark area pointed to by the arrows. The level inside the tank will rise to the top of the hash marks when completely warmed to operating temperature. That is the directions on page 263 below for fill instructions. If filled higher than the bottom of the hash marks, the level when warmed will be above which will not be particularly alarming.

By keeping the level when cold at the bottom of the hash marks, I have an excellent reference for the level of coolant so I can tell if there is even a slight loss of coolant.

The picture on page 263 is what my surge tank looks like when at operating temperature.

Caution: Do not loosen the surge tank cap unless completely cooled because the hot contents is under pressure. With level at the bottom of the hash marks, there is a good reference for safe removal of the cap.

 

Mine looks like The picture on page 263 when cold...

 

The usual GM coolant level in GM vehicles is right at the coolant tank seam. So with this vehicle, below that seems low…even if the the manual says it’s ok. So most GM techs will want to top it off to the seam and that’s fine too.
Well below the hash marks is definitely low but there is some gray area there where it “full enough” and nothing to worry about.
Don’t over think it.

 

Split the difference and go half into the hashes. Fuller than minimum, but not overfull and has room to expand.

Max

 

This is an example of how a poorly written instruction in a technical manual can cause a lot of confusion. The words say one thing, but the picture indicates something different. Fortunately, the ambiguity can be resolved by applying common knowledge and a good example.

Fluid tanks on automobiles and other equipment usually have two marks, FULL and ADD or LOW. This is true on coolant overflow tanks, engine oil dipsticks, etc. The XT5 coolant overflow tank has different notations. It uses a hatch mark area and an add mark (arrow). The top of the hatch mark area is the full mark. The problem is that the owner's manual doesn't clearly say that, hence the confusion.

The common knowledge aspect of this is that it is common practice that whenever you fill a fluid tank you fill it to the FULL level so if there is a loss of fluid during operation there is a margin of safety before the fluid goes below the ADD level. If you filled the fluid to the ADD level there would be no margin of safety for even the slightest loss of fluid. So, the words in the XT5 manual are misleading, but the picture is correct.

As an illustration and example of clearly labeled coolant overflow tanks, I have attached pictures of the engine and inverter coolant overflow tanks on my 2013 Toyota Avalon Hybrid. I hope this helps.

 

That's kind of how I interpreted the tank in the XT5...

 

Went to the dealership this morning for my appointment. On Tuesday the warning went away. It didn't come back until I was driving to the dealership this morning. They are replacing the thermostat under warranty. Didn't challenge me at all and the service person knew about the thermostat issues. There were some codes in the computer, but I haven't gotten a report on them yet. I won't get the XT5 back until Monday or Tuesday.

Also, no loaner or rental car available to give me. The service person said the chip shortage means they have very limited loaners/rentals. They did get me an Uber for the 30 mile trip back to my house though.

 

Can anyone provide the part numbers for the thermostat and sensor?

 

The dealership sent me a list of items to address. Was wondering if anyone could say how important/necessary they are. Thanks in advance:

• Flush hydraulic brake system
  • 3 years or 30k miles
  • $157
• Decarb Engine MOC
  • 30k miles
  • $286
• Replace transmission fluid
  • 45k miles
  • $576

They also recommended some other stuff:

• Service battery
  • Which is funny because I replaced the battery less than 2000 miles ago
  • Also, what exactly do they do here? They charge $48 so that definitely isn't replacing the battery...
• Replace wiper blades
  • Did that this year too
  • $71 isn't too bad for parts and labor on it
• Replace engine and cabin air filters
  • $65 and $83 respectively
  • I have not done this, but am sure I can do these myself for much less.

 

Those services are just what the chart or computer spits out at them to sell to you. For the most part they are good services that won’t hurt and can only help. Are they needed immediately…no but whenever you’re ready to. And obviously if your battery and wipers are good, don’t do those. Let them know the status. Maybe they can update the computer with that info. If they can’t then those recommendations will keep coming up.
Glad you got the Tstat replacement!!