Hyundai Sonata 2004 overheating


Asked by Dec 24, 2017 at 03:51 AM about the 2004 Hyundai Sonata FWD

Question type: Maintenance & Repair

I have the same problem with my Hyundai Sonata
2004 which is 4 cylinder engine. I bought this car a
month ago and lately it started overheating in stop
and go traffic. I watched a lot of videos and articles
replaced coolant temp sensor. Well this coolant
temp sensor has 3 pin outs 5 volt 12 volts and
ground. Anyway it did worked for sometime like first
run then started overheating again fans not kicking
on. I tested the coolant sensor it’s resistance going
down when hot and high when cold. It’s thermostat
is that lower hose. I was checking for thermostat
opening by running engine idle and looking at
coolant, no bubbles found but coolant flows out of
radiator for sometimes and drops down when it
gets hot but I don’t see any swirling or motion. Now
the car won’t overheating unless you run it for
awhile may be because it’s cold outside. Fan do
kick on when I unplug coolant temp sensor. I have
no freaking idea if this has a fan switch and temp
switch. Since fans turning on when I unplug I am
assuming all the electrical good. I ran the obd it
triggered P0118 code for temp sensor, this code
triggered after changing the temp sensor.Need help.

13 Answers


first see if the fans come on when it gets hot , if they don't see if they are getting power at the disconnect or hot wire them to see if they work , the plastic blades can get razor sharp so be careful , what is the code definition ? The sensor voltage sounds like the computer reads the high and low difference and the fans coming on when you unplug it may be a default program , the thermostat can be tested in a pan of hot water on the stove with a thermometer against the rating on it or just replace it with a quality part , that goes for anything you buy don;'t buy cheap junk , auto electric is usually a learning experience for me so I think you'll have to do more research if you want to do your own work , but that;s the purpose of life , to improve yourself

1 people found this helpful.

Fan don't come on when it gets hot. Fan turn on when I disconnect or turn on Ac so they getting power, if I am correct. Code definition coolant temp sensor circuit high. I think the voltage is for gauge because when I uplug the sensor gauge won't work.


right , the fans work , I would think the sensor is bad how many are there tied into the coolant lines ? , probably not that expensive , but it could be something else shorting out , not a tech myself

1 people found this helpful.

Its a new sensor. I willl replace and see. I took readings ground has 0.2 volts


I replaced thermostat even the old one still looks good. Flushed the coolant. Yesterday it seemed fine that fan were turning on automatically and shutting off. But this morning its was foggy snowing and while I was car indicator creeping up since I have an obd scanner I checked the coolant temp it was about 195 F I pulled over checked for fans. There were dead and not coming on. indicator reached all the up obd sensor reading 195 F. Is 195 F is the ultimate temp or my sensor bad again because its a new one ....

1 people found this helpful.

I'm not sure what any of it is SUPPOSED to do , 195 is a normal temp. for some thermostats to open ,you need a pan of water on the stove and a thermometer to test them , , it says on the thermostat what the rating is , why the fans weren't coming on when temp. was high I don't know ,


new thermostat is marked for 180 F. but fan is not coming on until 205F - I cannot say this confidently because sometimes it comes on even at 170 F - Most of the times it wont, anyway my temprature is creeping high at 195 this is a sensor issue right? Or is it because of thermostat because I recently changed it and its happening a day after.


if it's marked for 180 then that's the temp. it opens up at and allows the coolant to circulate, it does not control the fans that is done by the computer which gets info from sensors/input , if the fans come on at 205 I don't think thats a problem but if there are times they do not come on then it is a problem ,I'll be honest with you , those cars are crap not as bad a GM & chrysler but loaded with technology that is just not needed , The car is over 10 years old and is going to start falling apart , you may have to replace one thing after another soon

2 people found this helpful.

I am having the same fan issue with my sonata as well. Mine however is a v6 engine . Is it possible for it to be a fuse issue?

1 people found this helpful.

I just recently had the same problem that I am still in the process of dealing with, I don’t think it’s the heater core but I am still ruling that out even though I have light condensation under my drivers side floor mat, it seems that when peak summer season comes on the car is over working itself more, not so many problems during winter and fall, I changed the thermostat and thermostat sending unit, checked hoses no leaks, my coolant is always fairly cool with no steam, and there is perfect pressure in the system. there is no thermostat switch on this car the computer tells the fans to come on and in most Hyundai’s that is at 205 even though the thermostat itself opens at 180, I had my 23 year veteran mechanic diagnose it and he ran a temperature gun on it, it was the same temperature when the temperature Gage on the dash inside the car would get hot or level out at normal temperatures it said it wasn’t over heating at all, he said my dash meter is some how not gaging the correct temperature, he checked every possible thing and it all seems to be working, but it’s scary because it seems to always read normal temperature until I turn my ac on, and then says it’s overheating, idk just thought I would add my thoughts, when Tron said there was a problem with his heater core and noticed coolant on the floor how bad was it and could u feel the wetness up under the gas pedals because in my car it’s only under the mat in the drivers side but not under the dash

if im replace new engine, what engines is suitable for sonata 2004? anybody can help me? any suggestion?


Had the same problem about a month ago. It turned out to be a problem with temperature gauge wires and not with the cooling system. Everything was running under the normal operation temperature, despite the fact that the guage was indicating overheating. got the wires fixed and now guage shows proper temperature.

2 people found this helpful.

GuruBCZV8? Yes I had mine diagnosed as well and same thing my engine has never actually overheated. It is running under normal operating temperature. I changed the temperature gauge sensor still didn’t fix it. My mechanic told me it was most likely the cluster itself. I have literally been driving with it like this for 6 months with no problems but I would like to get it fixed. Where are the wires located so I can check them. How did you figure that out

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