Auto Repair Help

by Mark Davidson

The Engine Coolant Temperature Sensor (“ECT” for short) is an analog sensor which, as it’s name says, measures the temperature of the coolant and feeds this data to the on-board computer. The computer then uses this data to maintain optimum driveability especially while the engine is warming up and until it reaches operating temperature. In the days before electronic engine control modules, the choke on the carburetor served this purpose, ie. to cause a rich fuel mixture at least until the engine was warmed up. Frequently the engine had to idle at high rpm to keep it running when cold. Often it would stall before it got warm.

Coolant SensorEarly model OBD vehicles used a very simple circuit involving only a temperature switch (Fig 1). This switch was a heat sensitive (thermal) switch mounted in the coolant fluid to monitor its temperature. So long as the temperature of the engine coolant was below the engine operating temperature, this switch would stay in an open position. The 14 volts on PCM Pin D (14 volts which came from the PCM) communicated to the PCM that the engine hadn’t achieved spec operating temperature.

During a cold engine test, the PCM would retard the spark timing and run the engine’s fuel mixture slightly richer to help compensate for cold conditions which are so characteristic of any typical gasoline engine. The PCM constantly monitors Pin D so as to determine when the engine reaches operating temperature (Fig. 2).ECT When the engine reaches it’s operating temperature, the switch responds by closing the contacts & grounding Pin D. The voltage on Pin D drops to near ground voltage (almost 0.0 volts). The 14 volts supplied by the PCM gets dropped inside the PCM across the load resistor. As soon as the PCM reaches 0.0 volts, it immediately starts to change it’s fuel delivery by leaning the air/fuel mixture and advancing sparkplug timing so as to be able to handle a warming engine.

The PCM hence “thinks” the engine is stone cold even when it’s almost warm. But if the switch stays closed, the PCM “thinks” the engine is warm (even when the engine is just mildly warm). Specific control of air/fuel mixture and spark plug timing isn’t possible with a coolant temperature switch alone.

Later model engine coolant temperature sensors used something called a thermistor… a device which provides higher control over engine performance throughout the range of engine operations. So then, a thermistor is a variable resistor made of solid state materials that will change resistance according to temperature. The type of thermistor which is used in automotive applications has a negative temperature coefficient; this means resistance decreases as temperature increases.

Connected to Pin D inside the PCM is a 350 ohm load resistor (R1) connected to a 5 volt reference circuit. The resistor R1 and the ECT make a voltage divider of the 5 volt supply reference. Since R1 is fixed at 350 Ohms, the voltage at Pin D depends on the value of the ECT. So then, when the engine is cold, the ECT resistance is naturally very high. Some vehicles will use a ECT which has resistance of 100,000 ohms and yet others may be 50,000 ohms.

When the engine is cold, the ECT will have the majority of the resistance in the circuit and thus consumes most of the voltage. The resistor “R1”, only 350 ohms, drops only a slight voltage amount in comparison to the ECT. This makes the voltage on Pin D nearly 5 volts when the engine is cold. We can suppose that the ECT is 100,000 ohms (when cold). As the engine warm ups, the resistance of the ECT will begin to decrease. In the initial stage of engine warm-up, the ECT could be 75,000 ohms but “R1” is still 350 ohms. Thus the voltage of Pin D is still close to 5 volts. For the voltage of Pin D to drop to 2.5 volts will require the engine to get almost up to spec operating temperature so the ECT is also 350 ohms. Then “R1”, at 350 ohms, will drop half the voltage, and the ECT, at 350 Ohms, will drop the other half which makes the voltage on Pin D at 2.5 volts.

This is a rather simple procedure — watch the ECT voltage as the engine begins to warm up and then check the reading once the engine is at spec operating temperature. You should expect a reading from 0.8 – 1.2 volts for most vehicles. Always check a known good vehicle if possible to establish what a good ECT voltage (at spec operating temperature) is if the car manufacturer doesn’t supply such information. If the reading is higher than spec, the PCM will “think” the engine isn’t as warm as it actually is and thus enrich the fuel mixture and retard the engine timing which will make for warm driveability problems. In this case, you can expect higher emissions and severely reduced fuel economy.

A higher than spec reading will also be caused by poor ground at PCM, which the ECT is dependent on for proper operation. If the PCM ground is acceptable, you should check sensor resistance with a digital ohmmeter. Many manufacturers will provide a table of ECT resistance for a certain temperature. But if the reading is lower than spec, the PCM will “think” the engine is actually hotter than it really is and enlean the air/fuel mixture and also advance timing, which makes for cold driveability problems. In this case, expect difficulty in cold starting (of course). A lower than spec reading may also be caused by a defective ECT, or, the ECT signal wire to Pin D could be shorted to ground.

(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)

20 responses to “Coolant Temperature Sensor”

  1. Norris Fait says:


    I have a 2004 toyota carmery that runs like it should. Until it reaches operating temperature, then the car slows down to a top speed of 18 miles per hour. Is this the etc or is it perhaps another problem?

  2. matt parker says:

    having the same problem with a kia sendona diesel. been advised its the temp sensor. if any one has come across this please contact me

  3. aamir says:

    could be your ignition coils too! get those checked!

  4. Nikos says:

    Hello car is Freelande1 1.8 16v (18k4f)Y..2001 en,de radiator fan start on 105ce..en hier in Greece is not oke..en i niet 2 start on…nikos

    • Mark Webber says:

      Obviously your temperature sensors out of range on this vehicle. I’m not familiar with this particular vehicle so the temperature sensor might be on your radiator or it might be on your map sensor or on your inTake manifold. Get a Chiltons repair manual for your vehicle and it will tell you what range and what temperature sensors does what. From what I can see
      ECT or engine coolant temperature sensor is the problem and you should check that.

  5. never says:

    my altezza ect sensor cable was damaged cut while it was pessed to on position so nowm the engine becomes so hot and it drains the battery frequently then as i drive it cuts off

  6. Bob says:

    My car has poor heat at idle and good heat when driving.
    What causes that?
    Bob G

  7. ochiroo says:

    hi. i’m from Mongolia, pls help. i want to replace my ECT sensor. but it doesn’t have a thread… how can i remove old one? /sorry for my bad english/

  8. Jamie says:

    I had a bad engine coolant temperature (ECT) sensor with infinite ohm reading (open circuit) so the computer interpreted as cold engine temperature. Therefore the computer ignore the EGR valve assuming ice build up below 32F and there is an EGR malfunction.

    Now that I replaced the ECT, EGR valve is working sending exhaust gas into the intake manifold and I smell exhaust because my intake manifold gasket is leaking (I can see oil leak). Does this make sense?

    2000 Ford Taurus 24V DOHC

    • Mark Webber says:

      Intake manifold gasket leaking will never cause an exhaust smell. The car will run rough and the oxygen sensor will tell the engine to put more gas into the engine. This can cause your catalytic converter to overheat and it will possibly damage it soon. An exhaust smell might mean that you have an exhaust leak and maybe your Catalytic converter will be clogged. Check the auto parts store for something that will clean your fuel injection system and clean your catalytic converter. If your car is running louder than normal than I would suggest that you have an exhaust leak.

  9. My 2000 Chevy Blazer keeps knocking out the engine coolant temperature sensor. I’ve replace 2 of them. After about 2 weeks I notice the check engine light and see the temp gauge has quit working. When I have the diagnostics done, sure enough, it is the ECT. What else should I try?

  10. Ric Young says:

    Repeated ect failure suggests head gasket failure. Hot combustion gas bubbles released near the ect are hot enough to damage it. Had it happen on a Cat 3406B where the deck was so warped the block had to be milled to get a new gasket to seal. ECT would last maybe a week before it started to jitter, then one more week before failure. Check for combustion products in the coolant (oily surface, blackened appearance). Might also see high cooling system pressure before engine temp could produce it, unexplained coolant loss.

  11. Hugh says:

    I have a Mercedes V280 (Vr6 engine from VW) I have always had problem with too hot running. Changed the ECT (entire wiring harness actually)from another exact spare engine because this engine had two sensors into the thermostat housing and mine only had one (I thought one might have been removed at some stage). Ran well for a long time but after engine work (fix of auxiliary water pump)the sensor(s) is/are reading sky high temps that are not there in reality. Cold start issue is there too… Mercedes garages don’t even know this motor and technical information is contradictory. Should I go back to the one old sensor? But the duel was doing its work very well until lately. Both sensors have same cold resistance since I tested them cold. What should I do?


  12. […] Re: 2003 PT Cruiser GT Rad fan update…. The link below mentions checking the sensor resistance with a digital ohmmeter. In order to check your coolant temperature sensor we would need the table of ECT resistance for a certain temperature for that sensor. Coolant Temperature Sensor – Auto Repair Help […]

  13. herbert phillips says:

    were is the sensor located on a KIA

  14. Andy says:

    I’ve been having a problem for the past 6 months with my temperature gauge not working. I was told by a shop I could try unplugging the coolant temperature sender and measure the resistance across when hot and cold and see if a difference can be seen – and it was. So next I was told to check the plug that plugs into the sender by placing a wire onto the neg bat and the other on the block or frame, this would make the engine temp gauge peak out. I did this but nothing. I tried a few others and even measured the resistance across the plug and got almost 0, meaning it is a closed loop. I put the plug back on the old sender just to keep on going until I could figure out why my gauge is not reading. But now my engine light came on and when I brought it in to Autozone to check the code the guy told me it was two codes (I’m not completely wording this perfectly it is only what I remember from a few days ago): 1) engine temperature is low or low input, maybe it was high as noted here, and 2) either this one was high input or high voltage reading. So is it plausible I shorted something out in the PCM? Now my Ranger is running as if it is cold from the way it sounds.

    Any help on this I would very appreciate as every autoparts store I’ve gone to so far has been crap.

    Thank you

  15. Harold says:

    Installed a new radiator and fans in my 2003 Toyota Avensis Verso and now the radiator fans only runs with the a/c on. The fan is not cutting in as the temperature increases. What can be the problem?

  16. ian says:

    how do I make coolant temp sensor believe temp is hotter thermostat bypassed resulting in running rich when cold want to fool ecu that its hotter

  17. Jean Hogge says:

    My garage told me I needed a new Throttle Body fuel injector dealy (Chevy 350 in a 94 Blazer), way too expensive OMG, I talked to an Auto Zone man and he suggested I try changing the coolant temperature sensor which cost about $17. Guess what? My rough idle (sounded like it had lost a cylinder or two) that was happening on a restart disappeared. AMAZING.

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