My digital engine temp reading was at normal operatIng range (199-203) then all of a sudden dropped to 148/149 which brought up the cold engine yellow and red lines, then after about 5 minutes jumped immediately back to normal operating temperature 199. Could this be a faulty coolant temp sensor? Dealer just replaced the thermostat approximately 50 miles earlier because the engine temps would not go above 170. That seemed to fix that however now this.

Any suggestions will be greatly appreciated.

 

The car has a lot of software and tech . It could be a software bug that will remain a mystery unless the problem is easily repeatable

 

I’m having a similar issue. I was running normal temps and all of a sudden started getting readings so low that they almost appear to be in Celsius. I run with the Gauges in track set as a norm so decided to switch to sport and found temps to be reading normal. I’ve posted about this bur have only received minimal feedback but no real info. Not sure what to do and dread the thought of taking it to the dealer. Anybody ???

 

If it’s still under warranty take it to the dealer and even if they can’t figure it out you have it documented so if something bad happens after warranty is up you have a case to get this issue covered. Unless it is easily and readily repeatable I don’t think much can be done.

 

Possibly 2 different scenarios for the OP, only one that applies to YOU since you didn’t mention your cooling system being serviced recently.

1. I have seen this exact scenario several times after a thermostat replacement or draining the coolant system for some other repair.
The cooling system has a pocket of air trapped in the engine block or somewhere in the cooling system.
The refill procedure for the coolant is a step by step process that many mechanics don’t follow.

Seven times out of 10 there won't be an issue, but other times you get air trapped in the system and it causes the temperature sensor not to be completely submerged 100% of the time and/or it encounters coolant of widely varying temperature.

The reason for this is during the fill process the thermostat is closed and the mechanic fills the system to capacity thinking its full however the void left in the engine block can't be filled since the thermostat is closed. The mechanic or owner then runs or drives the car normally. But guess what?, the temp gauge isn’t accurate.

The gauge shows 1 or 2 things. One being the car overheating or the other being the car is really cold and not warming up.
Either of these scenarios is caused by the same thing, that being a trapped air pocket.

A. The overheating condition is caused by the air pocket being located on the backside of the thermostat, therefore the thermostat being a wet bulb never gets to its opening temperature since it isn’t wet.
This results in the water in the engine block overheating as it never leaves the block to circulate through the radiator because the thermostat is closed.

B. The air pocket gets trapped in the block or the water level in the block is so low as to not cover the temp sensor or only splash on it from time to time.
This results in (what I believe the OP is experiencing) an erratic gauge.


2. The temperature sensor could be bad. BUT the temperature sensor has fault logic in the PCM to detect when it is shorted or losses ground.
Because of this logic the probability of the sensor or sensor wiring being bad on a fairly late model vehicle is lower.
Simply because if it was not responding you would get a code indicating a faulty temp sensor.
This fault circuitry does not cover 100% of the possible failure modes so there is a small possibility the sensor is bad regardless (This can be determined fairly quickly by monitoring scanner data real time).

IF you haven't had any cooling system issues or services causing #1 above AND you positively know your thermostat is good then chances are the T-sensor is YOUR issue.

HOWEVER If the thermostat isn’t known to be good then a thermostat would more than likely be the cause as it may be operating erratically before it completely fails closed, partially open or fully open.

Therefore your recommended course of action if #1 above isn’t a possibility is to change the thermostat first, since it has no direct way for the PCM to know it has failed.

Then if required change the temp sensor since 75% of the possible temp sensor failure modes are monitored by the PCM (betting with the odds and guessing the percentages here).

I know you said your readings are normal in modes other than track.
However some readings in the displays are filtered feeds based off of an initial feed to the display. This means it is possible if what you think is "normal" in the modes that read normal are even slightly off or erratic they can throw the reading off in Track mode by much larger percentages than we would think.
My point is unless you can monitor temps with a scanner and verify they are in fact 100% normal and not slightly off in the mode where you think they are "normal" then all bets are off for the other modes. Unless your gauge cluster is bad, again unlikely if we are playing the odds.

The cooling system is pretty simple.

On a fairly late model car unless there is a leak or a fan problem the culprit is either the T-stat or the T-sensor IF we know the coolant service and fill procedure was done correctly and there is no air in the system.

Good luck.