I had this on a 2010 XK (same mechanically as WK), happened on very cold mornings below freezing. I tried a new thermostat and then a new Temp Sensor, both seemed to improve the situation, but it would come back. I also had an intermittent coolant leak, and noticed it happened when the coolant level dropped.
I can understand on how being low on coolant could cause overheating/undercooling; but how does it cause overcooling? Best I can figure, being low on coolant would result in the engine not warm-up evenly, some spots getting hotter while others stay cooler. And the temp sensor is near the top of the engine, where if you're low on coolant it's going to circulate less at the top of the engine, so that makes sense.
This was the 3.7L V6, different than the newer 3.6L V6. But my problem ended up being a minor, intermittent headgasket leak. I used some BlueDevil Headgasket Sealant, it really works. Not the "correct" fix for a headgasket leak, but it worked, I stop loosing coolant and I stopped getting the P0128 codes.
Mine came back on yesterday driving down the highway with it being 70 or so very humid and therefore A/C was on. I don't use A/C much so not sure if tied to that or no. I might try thermostat as well soon. Let me know how it goes. I have 88k on car.
Ummm, you know the AC being on dumps heat out of the Condenser into the Radiator, if anything, the AC being on should help the motor warm up sooner and will cause the engine to run hotter.
I stopped to help a woman stuck on the side of the road once. She had overheated her car so bad the radiator burst. She said the engine was running hot, so she turned on the AC to cool it off, and then things went really bad.
Oh really, you think?
P0128 sets when it takes longer than normal for the engine to reach operating temperature. Generally caused by a thermostat that is partially stuck open and/or low engine coolant.
If there is a problem with the coolant temperature sensor, there are several trouble codes that can set for that as well.
This was added to OBDII for emission purposes. When the engine is cold it makes more pollution, so if the engine takes too long to warm up its sets a MIL to get the owner to repair it and keep the emissions down.
Now the vehicle was designed and tested to meet these OBDII requirements, but in this case, on cold mornings it's so close, that the slightest thing being off can cause it to take a few minutes longer than it should, and the code is tripped.
So to add to what Ratchet said, the problem is the engine is taking longer than it should to warm up. At least where the sensor is reading the temp. In the past, no one would care, and it wouldn't be a problem unless it was a symptom of something worse and if that was the case other symptoms to come up eventually.
But today, since an engine taking too long to warm-up would produce additional emissions, it sets a MIL. Regardless, its a sign that something is wrong, but what's frustrating it could be the result of something just barely being off, which makes it difficult to identify.
I just had this code read at AutoZone. P0128, THERMOSTAT RATIONALITY. 2011 WK2 with V6.
Here is my situation: I have MaxCare warranty from Chrysler and already have an appointment with dealer on May 7 (3 weeks from now) for "no heat coming from the vents on the driver side" and 2 recalls. This is the earliest appointment I could get with a loaner car (I must have a car to get to work)
Is it safe to drive (40-45 miles/day) for 3 weeks? Other options include going to my local mechanic and pay out of pocket (I'd rather not) or try to get an earlier appointment with this or another dealer.
You do know what the number one cause of no heat coming out of the vents? Being low on coolant. If the coolant level gets too low it won't circulate through the heater core and thus you won't get any heat. As well, only a little coolant circulating through the heater core will cause only a very little heat. The Modern HVAC systems under the dash are pretty compact, and the air has to make some tight bends. So when the AC or Heat gets really weak, the tight bends often result in air that's heated or cooled making it to one side of vents. As well, leaks can cause only part of the heater core or evaporator to heat/cool and thus air for one side coming out on one side not being heated or cooled. If this is the case, if one side has no heated air, then the other side might have heated air, but its barely heated at all.
If you have a dual climate system, it might simply be the mixing doors for that side got jammed or failed. If that was the case, then the side with heater/cooled air would have fully heated or cooled air.
Since this "could" be a symptom of being low on coolant, why would you wait 3 weeks to check your coolant level?
Keep in mind, the "proper" way to check your coolant level is when the engine is cold, you remove the pressure cap from the radiator and make sure the coolant comes up within an inch of pressure cap mount.
The reservoir (overflow tank) only shows the coolant level if the cooling system is working properly. The manufacturers recommend checking the level at the reservoirs because of liability, to many idiots burn themselves badly opening hot radiators. If there is a leak in the system, when the engine cools and coolant contracts, it will suck air back in through the leak instead of sucking coolant back in from the reservoir. I have seen plenty of cars where the reservoir indicated the cooling system was full, but the cooling system was almost empty.
So check the coolant system level properly, only when its cold, and fill it if its low, make sure to use the proper recommended coolant. Beware the "All makes, all models" crap on the store shelves, which seems to be the only thing you can find anymore. This stuff is crap and not compatible with other anti-freezes, they've been sued for making the claim, and lost, but still keep making the claim.