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).