Hi guys,

I have a bit of a problem with my coolant temp sensor circuit.

My car threw a check engine light and error code "P0115 Coolant Temperature Sensor Circuit Malfunction" earlier. I replaced the sensor itself with a brand new item, and checked the wiring etc for any potential ground and came up with nothing.

The new sensor didn't solve the problem, and when I try and clear the fault code, it just returns immediately.

I've checked out the temperature sensor values on MultiEcuScan and the coolant temp sensor reading seems to be ok - started off around ambient and gradually rose to 70 odd degrees C, all whilst the engine light remained and wouldn't clear.

What is strange though is once the car is up to temperature and the coolant temp gauge is at half way, if I turn the car off and straight back on, the coolant temp gauge resets itself to zero and takes a while to rise back to mid way - but the reading on the diagnostics is still showing the correct figure. Usually the temp gauge would just return to the correct half way point on the gauge

The car feels like it's in a safe mode as it's definitely down on power. It struggles a bit to start when warm too, which to me seems like the engine may be over fuelling as it thinks it's doing a cold start.

Anybody have any ideas what this might be? Could it be ECU related? I'm at a bit of a loss.

Thanks.

FDR said:

Hi guys,
seems like the engine may be over fuelling as it thinks it's doing a cold start.

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So I guess it's a petrol engine; which one ?

FDR said:

What is strange though is once the car is up to temperature and the coolant temp gauge is at half way, if I turn the car off and straight back on, the coolant temp gauge resets itself to zero and takes a while to rise back to mid way - but the reading on the diagnostics is still showing the correct figure.
Anybody have any ideas what this might be? Could it be ECU related? I'm at a bit of a loss.

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How much is "a while"??? 1-5 seconds or 20 minutes????????

Bernievarian said:

So I guess it's a petrol engine; which one ?

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T-Jet.

aurick86 said:

How much is "a while"??? 1-5 seconds or 20 minutes????????

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5 minutes or so driving normally - basically as if the engine has started again from cold - when in reality it's still at normal operating temp when re started.

that is not ok,,, mine (t-jet Bravo) goes to actual temperature in 5-10 seconds)

I suggest finding the wiring diagram, check to see if maybe some other sensors are using the same power or ground wires (could be joined inside the ECU as well). Measure the voltages. usually 5v power to sensor.

Check the diagram to see if there are any other connectors between the sensor and ECU. Spay them with contact cleaner.. add some dielectric grease.

Find the pins for the temp sensor in the ECU connector:
Check resistance with an ohmmeter on the wires with the sensor connected.
A thermometer will be needed as well to check the voltage to temperature correlation.

Lastly what is the intake air temp?
I'm thinking maybe the ECU is reading that temp as well and it's somehow confused as to which one is reading abnormally.

On my T-jet the engine coolant temperature usually sits at around 75 °C, that is when the thermostat opens... goes to like 85-88 and then the fan starts. ( this is from memory..might be slightly off)

Are you 100% SURE you replaced the right sensor ? The K036 is located on the thermostat housing, under the airbox reinforcement bracket, gearbox side. It's a basic NTC fed with 5VDC for the engine control unit; it sends back the signal to the ECU for processing. The K036 is the only sensor involved in dashboard temperature indicator.
aurick86 : your thermostat should start opening @ 88 ±2°C , that allows the engine to reach 90°C, your fan should NOT start at such low temp ! BTW, you say that it usually sits at 75°C, that would indicate a non-working thermostat, stuck (partly) open

Here are some drawings with pins # ...

Regards, Bernie

If someone here helped You fix or understand your issue, hit the thanks icon in lower right, it's free and make us proud...

Bernievarian

I think i was not clear there:
-the thermostat starts opening at 75°C, but only slightly without load the temps will rise.. but slow.
-with load it rises much quicker
-at about 88°C the fan kicks in on the first speed so it would make no sense that the thermostat open at the same temperature.

P.S the gauge in the car shows the perfect temperature indifferent on the +/-10 degree variation from perfect temperature.

I thought so too that's why i replaced it... same thing (replaced with Gates stated temp is 80°C).

The 88°C thermostat is for the aspirated 1.4 engine (the thermostat housing is slightly different- there is no pipe going to the turbo..and maybe some other difference)

I did search for the hotter thermostat 88°C just thinking it will get the mpg slightly lower, and keep the spark plugs cleaner.. did not find one for the t-jet .. only the 80°C one.

Bernievarian said:

Are you 100% SURE you replaced the right sensor ? The K036 is located on the thermostat housing, under the airbox reinforcement bracket, gearbox side. It's a basic NTC fed with 5VDC for the engine control unit; it sends back the signal to the ECU for processing. The K036 is the only sensor involved in dashboard temperature indicator.

aurick86 : your thermostat should start opening @ 88 ±2°C , that allows the engine to reach 90°C, your fan should NOT start at such low temp ! BTW, you say that it usually sits at 75°C, that would indicate a non-working thermostat, stuck (partly) open

Here are some drawings with pins # ...

Regards, Bernie

If someone here helped You fix or understand your issue, hit the thanks icon in lower right, it's free and make us proud...

Click to expand...

Thanks Bernie, useful diagrams.

Definitely replaced the correct sensor on the thermostat housing.

Like I say, it's really odd because when i'm checking on MultiEcuScan it appears that the sensor is working fine, the engine temperature value rises normally and isn't stuck showing an extremely low or high temperature like I'd expect with a broken sensor.

The fault code just will not clear, and the check engine light is permanently on as a result.

What does MES see if you unplug the sensor?
Maybe it's giving you a calculated temperature somehow.

@ Aurick86,

strange thing this thermostat, eLearn says 88 +/- 2°C for all models ...
Display shows permanent ideal temp (when in range) because the probe signal is processed in the ECU to do so (for display only). The fan speed is also managed by the ECU, but couldn't find with what strategy values, first speed @ 88 seems low to me still, but as you said it wouldn't make any sense to start the fan while the thermostat is just opening...

Regards, Bernie

Tjet engines use 80c thermostat.

Low speed fan activates at 97c
High speed at 102c

(There is also a coolant pressure sensor for safety reasons)

Hi CMOS and All,

first time ever I hear about a coolant PRESSURE sensor ! Can you show in the drawing where it is ??
I believe they are TWO pressure protections on a modern engine:
- one for freezing coolant: little insert in the block that would pop-up to release water expansion due to icing (water expands below and aboce 4°C). This is to avoid the block from splitting...
- one for excess pressure in the radiator (or expansion tank) cap: this allows pressure higher than atmosphere to increase the water boiling point. At higher pressure, the coolant will NOT get bublles (they start forming @ 80ish°C @ 1bar), this is to avoid cavitation issues in the pump...

Never heared about other device...

About the thermostat: I've cheked yesterday with MES during a 180km journey: water temp oscillates around 83°C by about 2°C, highway/city/country side ; I'd interpret this as starting opening at 80 and full open at 88 ?? That would made us all agree ;-)

Regards, Bernie