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Yaris Low Coolant Temp Problem!

Hi there all. Before someone says “normal” I know the low temp light is supposed to come on until the car reaches normal operating temp :slight_smile:

My problem is really strange and my mechanic and I are both perplexed. I have a 2008 Yaris automatic sedan with 98K miles on it.

The symptoms:

After driving for quite a while (30 minutes or more at times) the low temp light will come on… and sometimes goes right out… and sometimes goes on for a while.

If the check engine light isn’t already on - it will eventually go on.

Sometimes with the low temp light is on - it will race the engine (just straight high rev) OR it will pulse the engine (like I am pumping the gas peddle) OR it will not let the car shift into high gear (rare). Remember… all of this happens after the car has been running for while so it warmed up.

What we’ve tried (over a 4 week period now):

The error code indicates to replace the thermostat - so we’ve replaced it… twice
We’ve replaced the actual low temp sensor
We’ve checked the wiring between the sensors and the computer - wiring does not appear to have a short
Last week we replaced the computer (thank GOD with a used one (200.00)…because the same problem still exists with a replaced computer… a new one would have cost 800 - 900 dollars).

What the mechanic says now:
Might be an air conditioning thing running the fan too long?

Can you really run a fan so long it will drop the car temp back to below the low range operating temperature while driving on a highway? That seems pretty unlikely to me…

Has anyone seen this? Does anyone have any ideas what we should try next? If it were just a flickering light, I wouldn’t be so concerned… but because it revs the engine (even when I am at a stop light and I have to push the brake harder so it won’t start moving) and it sometimes wont shift into high gear… it’s concerning from a safety perspective.

Please Help!

“If the check engine light isn’t already on - it will eventually go on.”

Do you know if any DTCs are set when this happens and can you tell us specifically what the code(s) is / are ?


Have Your mechanic Take A Look At A 2007 - 2008 Toyota Yaris 4-Page TSB (Technical Service Bulletin) T-SB-0344-08, Written For Their Technicians.

See if it looks like your car needs a new improved Engine Wire Harness. You’ll need to check your car’s VIN against the bulletin.

Possible codes that will result from a faulty harness are P010_, P0121, P0125, P0172, P035_, P2237, or P2238.

That PO125 - “Insufficient Coolant Temperature for Closed Loop Fuel Control” is the one that caught my eye.

Take a look at this forum. I wasn’t logged in so I couldn’t open their copy of this bulletin that’s posted here, but maybe you could log in and scroll down and find it and have a look yourself.

This could be your problem or possibly it’s not it. Check it out with your mechanic.

Hope this helps.


Next time the low temp light comes on pull over and see if the cooling fan is running.
What does the temp gauge on the dash (does it have one?) read when the light comes on?

I have a similar problem going on with my '03 Camry. Except, it doesn’t have a separate sensor for low temp. It has a temp switch and a temp sender. To the best of my knowledge, the switch is used for over temp fail-safe and the sender is used to measure engine temp for the gauge and ECM. The specific code eludes me now but in effect, it says the coolant temp is below regulating temperature. It is a timed test that after some period of running, if the temp sender does not measure and report a value that exceeds the expected value for a 180 degree thermostat, it sets a code.

I bought a sender and thermostat but haven’t yet replaced them. Both were relatively cheap insurance. The other possibility is that there is an air bubble in the system. If the sensor probe is not immersed in coolant, it won’t read properly and will likely appear too cool compared to the coolant.

After all your mechanic has done, I would try to make sure you have burped the system properly to insure there isn’t trapped air causing an incorrect reading. As a final note, when looking up parts, I noticed they sell an OEM temperature thermostat at 180 degrees and also 170 degree thermostats. Why a 170 is offered, I do not know but there’s a good chance if you installed one (by mistake?), it might read too low for the computer setting and continue to generate the code even though you replaced a worn out t-stat with a new one.

Thanks CSA I’ll check with my mechanic.

Circuitsmith - no temp guage on dash (that would be awesome wouldn’t it? :slight_smile: )

Twin Turbo -

Good thought on the thermostats - I’ll check. The problem is, the issue was happening before I replaced the thermostat… so while, I imagine the symptoms would be similar, it would seem to be a pretty amazing coincidence that the syptom would look exactly the same if he installed the wrong one (my mechanic did the work).

Hi, buckaye, did you get to fix the problem?

I’ve had the exact same symptoms in my 2014 Yaris, and haven’t had luck with the thermostat, the sensor or the wiring.

This has happened for more than a year, so I’ve had some time to test some things. The most important thing I noticed is the change in temperature, I tracked it while driving and noticed that temperature seems to “drop” and “increase” suddenly, almost as if the sensor stopped working for a while (it can be a second or ten minutes).

Edit: you can realize in the picture that revs went down immediately as the temp reading went back to normal and the car stopped trying to heat itself up.

Does anyone have any idea what can be happening or what can I try?

Thank you!

If the horizontal scale is in minutes or seconds, and the temperature in deg C on that display (shame on them for not showing the units) , well, the coolant couldn’t actually change temperature that fast, so it must be a problem with how the computer determines the temperature; i.e. sensor, harness, or computer. Try disconnecting and reconnecting the sensor and see if the display jumps the same amount. If so, good bet there’s a bad connection somewhere between the sensor and the computer. Note that a ground path could be the source of the bad connection too. My guess is you got some sort of corrosion on a connector in that path.

There was a recent discussion on corroded connections for light bulbs here recently, and it seems there isn’t much that can be done about it other than just cleaning the bad connection when it happens. Note that the “corrosion” can be nearly invisible, just a thin oxide layer that requires scraping or erasing off. If I had that problem and couldn’t figure out why I’d be tempted to just run a separate wire from the sensor to the computer and see if that solved it.

Thank you very much for your reply. The horizontal scale is in seconds and celsius, so it is impossible for such change in temperature to actually happen.

What I’ve noticed is that it always changes to the same value when it goes down (aprox. 10 degrees). I’ll check if that is the initial value when the engine starts, to see if it’s the same as “actually cool” or if that is a “default when disconnected” value.

I haven’t really have all the wiring cleaned, so corrosion might be the right call. I’ll try to clean and reconnect everything and see if anything changes.

What baffles me is that this is somewhat of a “seasonal” behavior. When it happens, it keeps happening for several weeks, but then it can go for months without happening again. I’ll try to notice if passing over a pothole or something similar has anything to do with the start/end of the symptoms, as that might indicate bad contact or corrosion.

Thanks again, I’ll try your suggestions and see if something changes.

Common thing for iffy electrical connections. I have a battery operated wall clock located on an outside wall that consistently stops working when the outside temperature is lower than 40 degrees. It was unusually cold for San Jose, in the 30’s , last night in fact, and I had to remove & re-install the clock’s battery this morning b/c when I woke up at 7:30 am the clock said 3:15 am.