Overheating Warning, But Engine Temperature is Normal – Citroen C3 1.4

[ismlyldz]

Hello,

My car is a Citroën C3 1.4 Petrol, model year 2002/2003.

I’ve been experiencing a false overheating issue. The temperature gauge normally stays at the 3rd bar, but sometimes (especially in hot weather) it suddenly jumps to the 6th bar, triggers the warning, and then drops back to the 3rd bar within 3–5 seconds.

The jump is instant — it goes directly from 3 to 6 bars, and back from 6 to 3 bars just as quickly.

During these jumps, there’s no boiling coolant, and the radiator fan is running. When the warning occurs, I can hear the fan switch to its 2nd speed.

This never happens in cooler weather. In hot weather, it can even occur after 20–30 minutes of idling.

Initially, I suspected the temperature sensor or something related to the coolant. I took it to a mechanic, and we replaced: timing belt, water pump, coolant (with antifreeze), filters, and the temperature sensor (OEM). However, the problem remains.

I’m unsure which steps to take next.

(The only change I made shortly before this issue started was replacing the battery. The problem appeared 1–2 weeks after that, but I didn’t drive the car much in between, so I can’t directly link it to the battery replacement.)

Any advice or diagnostic direction would be appreciated.

[mickthemaverick]

After replacing the battery did you carry out a full BSI reset, during which the ECU relearns all the sensors?

[ismlyldz]

After replacing the battery did you carry out a full BSI reset, during which the ECU lrelearns all the sensors?

No, I haven’t. I have no idea what it is. Can I do it myself, and how?

[Paul-R]

Reset procedure here

viewtopic.php?p=601679#p601679

[PaulC5]

We had a 2003 1.4 petrol C3 and that did this a couple of times a year apart but in freezing conditions on a motorway. No faults found. When I replaced the timing belt, water pump, coolant, I also replaced the thermostat and it did not happen again. When it happened I put the heating on full and opened a window so the temp indication fell back to normal. I believe on this engine there are 2 coolant temperature sensors so you should also check the second one as well as wiring/connectors. I did not replace any sensors.

[ozvtr]

Unfortunately this is fairly common for the TU3 engines.
The engines (may) have 2 temperature sensors. A "green" sensor located on top of the thermostat housing which is used by the engine ECU to determine the coolant temperature. And a "blue" sensor under the thermostat housing to monitor the cylinder head temperature. The blue sensor is a back up and sets an alarm (all the temp LEDs come on) when the cylinder head temp reaches 120'C (in theory).
The green sensors are notoriously unreliable. However, the blue sensors were removed from the production in (about) 2005. I wonder why?
To test this theory you could just simply disconnect the blue sensor. It is normally an open circuit switch that goes short circuit, to ground, at 120'C. So "normally" it's not connected to anything.

[ismlyldz]

Unfortunately this is fairly common for the TU3 engines.
The engines (may) have 2 temperature sensors. A "green" sensor located on top of the thermostat housing which is used by the engine ECU to determine the coolant temperature. And a "blue" sensor under the thermostat housing to monitor the cylinder head temperature. The blue sensor is a back up and sets an alarm (all the temp LEDs come on) when the cylinder head temp reaches 120'C (in theory).
The green sensors are notoriously unreliable. However, the blue sensors were removed from the production in (about) 2005. I wonder why?
To test this theory you could just simply disconnect the blue sensor. It is normally an open circuit switch that goes short circuit, to ground, at 120'C. So "normally" it's not connected to anything.

I don’t know if there is a blue sensor that measures the cylinder head temperature, I will check it at the first opportunity.
What I am curious about is: when the coolant temperature is around 90 degrees, could the cylinder head really be 120 or higher? If so, what kind of fault or issue would this indicate?

I heard from someone who experienced a similar problem that after changing many components, they were able to solve the issue by replacing the fan relays. Could this be possible?

Thank you for your answer.

[ozvtr]

If the engine has a genuine cooling problem, the engine temperature will tend to creep up and stay there. Then come down slowly as the engine cools down.
For the temperature to "jump up and down" it's most likely a sensor problem. Particularly if there are no signs of a cooling problem, like bubbles in the header tank or the tank boiling over!
If the lower radiator hose is only warm to the touch, the radiator is working fine. If the upper radiator hose is hot to touch, then coolant is flowing out of the engine and the thermostat is open.

[ozvtr]

The "blue" cylinder head temp switch is actually called the "low coolant level sensor" but it's not actually monitoring the coolant level!!!
The theory is; that if there is NO coolant in the thermostat housing the "green" temp sensor will only read the temperature of the vapor in the housing (maybe 100'C?). Meanwhile the engine is actually over heating because there is no coolant! But the blue sensor will trip when the cylinderhead gets to 120'C.
however, as I said, the blue sensor seems to have been discontinued around 2004.

[ismlyldz]

If the engine has a genuine cooling problem, the engine temperature will tend to creep up and stay there. Then come down slowly as the engine cools down.
For the temperature to "jump up and down" it's most likely a sensor problem. Particularly if there are no signs of a cooling problem, like bubbles in the header tank or the tank boiling over!
If the lower radiator hose is only warm to the touch, the radiator is working fine. If the upper radiator hose is hot to touch, then coolant is flowing out of the engine and the thermostat is open.

It definitely jumps from stage 3 to stage 6 in a single movement, and then back down from stage 6 to stage 3 in a single movement, and it takes a maximum of 5 seconds.

I haven’t observed any drop in the coolant level, and I even had it replaced, but nothing changed!

I agree with your thought that “the temperature jumping up and down is most likely a sensor problem,” but even after replacing the original temperature sensor (the green one), the issue didn’t get resolved.

I haven’t changed the thermostat yet—could the thermostat cause such a problem? Could it be something instantaneous?

I heard from someone who experienced a similar problem that after changing many components, they were able to solve the issue by replacing the fan relays. Could this be possible?

Thanks.

[ismlyldz]

The "blue" cylinder head temp switch is actually called the "low coolant level sensor" but it's not actually monitoring the coolant level!!!
The theory is; that if there is NO coolant in the thermostat housing the "green" temp sensor will only read the temperature of the vapor in the housing (maybe 100'C?). Meanwhile the engine is actually over heating because there is no coolant! But the blue sensor will trip when the cylinderhead gets to 120'C.
however, as I said, the blue sensor seems to have been discontinued around 2004.

According to your theory, if the "blue" sensor is present and faulty, I could experience this problem.
Another thing I understand is that if the blue sensor is present and not faulty, then an engine that has reached 120°C definitely wouldn’t suddenly drop back to 90°C.

(By the way, when I check the VIN number online, it shows the model year as 2002!)

I wonder if I’m right.

Thanks.

[PaulC5]

Jumping up and down so quick could be a wiring problem, check the sensor wiring for any damage that might be touching something metal and shorting to earth. If you have a code reader with live data see what temperature it is showing and maybe get somebody to move the wires about but be careful in case the fan starts up.

[ozvtr]

Check under the thermostat housing for the blue sensor. You will need to take the air filter housing out to get to it.
If there is one (there should be if it's 2002) disconnect the blue electrical connector, replace the air filter housing and see if that solves your problem.

[ismlyldz]

Check under the thermostat housing for the blue sensor. You will need to take the air filter housing out to get to it.
If there is one (there should be if it's 2002) disconnect the blue electrical connector, replace the air filter housing and see if that solves your problem.

Yes, I found the sensor you mentioned. When I replaced it, no fluid leaked during the process.
I’ve driven the car a bit since then, and it seems like the problem is resolved.
Thank you very much.

[Tamz]

For anyone with the same problem. I have a 2018 Relay van and it started doing this around 3 weeks ago. As i could always keep it going and had local work commitments, i had to bear with it.
What i noticed over that time.
The gauge would shoot up, warning light on, van jerked, but kept driving, the gauge dropped within a few seconds. It could happen 100m away from a cold start. It would do this twice, then the third time the engine management light would come on, it would slightly stall but keep going if i dipped the clutch and revved it. 4th time, the engine would cut out, management light on, and sometimes telling me engine fault, please stop and i would need to dip the clutch, coast and restart on the key. Faults cleared themselves.
After around 2 weeks of this i realised if i kept my foot partially on the clutch it would be fine.
Finally got it in the local garage this afternoon and the guy had it hooked up to his machine and i mentioned if i kept the cluch slightly depressed, it would keep going.
He said it has a clutch sensor fault in the memory, so he disconnected the sensor, and immediately the gauge jumped up.
He did a few other tests and said it needs a new sensor.
Onto his wholesaler, part delivered within an hour, fitted in 15 minutes and the van now drives fine, hopefully problem solved.