Question about 2001 Pontiac Grand Prix

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Check engine light intermittently on

Diagnostic reading indicated, coolant temp is lower than thermostat. Should I first replace the thermostat?

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  • Pontiac Master
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When this happens, you usually need to replace the temparature sensor.

Posted on Dec 13, 2008

  • Ginko
    Ginko Dec 13, 2008

    This happens when the Engine Coolant Temperature (ECT) does not reach 75°C (167°F) despite sufficient engine warm-up time.

    This happens usually when the thermostat/temperature sensor is faulty, and is bad for emissions and fuel consumption.

    The repair cost may vary, even if themperature sensor is not an expensive part, after replacing the sensor there is need to reset the ECU (car computer module).


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May 12, 2016 | Cars & Trucks

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I have 2006 Chevy impala 3.5l and it's slightly overheating and I just recently changed the water pump and thermostat but it didn't change, and when I turn my heater on the air flow is very low.


check the lower (suction) radiator hose that it cannot collapse, they usually have a metal coil inside to prevent this. If you left this out when installing the water pump it could be the cause..What temperature is the gauge reading? You could have been running too cool before and now that the new thermostat is in its up to normal temperature.

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Jun 27, 2015 | Cars & Trucks

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P0128 (and P0116) on 2006 GMC Sierra v-8, 5.3L, 4x4. Originally getting low temp readings on guage. After replacing the thermostat, the problem seemed to be fixed. A few days later, I was getting my...


P0116 - Engine Coolant Temperature (ECT) Circuit Range/Performance Problem
The ECT (Engine coolant temperature) sensor is a thermistor which changes resistance based on temperature of the coolant in contact with it. The ECT sensor will be located in the block or coolant passage. Usually it is a two wire sensor. One wire is a 5 Volt supply from the PCM (Powertrain Control Module) to the ECT. The other is a ground to the ECT.

As the temperature of the coolant changes the resistance on the signal wire changes accordingly. The PCM monitors the reading and determines coolant temperature in order to perform essential engine fuel management. When engine coolant is low, sensor resistance is high. The PCM will see a high signal voltage (low temperature). When coolant is warm, the sensor's resistance is low and the PCM will see a high temperature. The PCM expects to see slow resistance changes on the ECT signal circuit. If it sees a quick change in voltage that isn't consistent with an engine warming up, this P0116 code will set. Or if it sees a lack of change in ECT signal, this code may set.

Symptoms: There may be no noticeable symptoms if the problem is very intermittent, however the following may occur:
MIL (Malfunction Indicator Lamp) illumination
Poor drivability
Black smoke at tailpipe
Poor fuel economy
May not idle
May exhibit stalling or misfires

Causes: Potential causes of the P0116 code include:
Missing or stuck-open thermostat
Bad ECT sensor
Short or open on signal wire
Short or open on ground wire
Poor connections in wiring

Possible Solutions: If there are any other ECT sensor codes, diagnose them first.
Using a scan tool, check the ECT reading. On a cold engine, it should match the IAT reading or should be equal to ambient (outside) temperature reading. If it does match the IAT or ambient temp, check the freeze frame data on your scan tool (if equipped). The saved data should tell you what the ECT reading was when the fault occurred.

a) If the saved info indicates that the engine coolant reading was at the coldest exreme (around -30 deg. F) then that's a good indication the ECT resistance was intermittently high (unless you live in Anchorage!) Check for an open in the ECT sensor ground and signal circuits and repair as necessary. If they appear okay, warm the engine up while monitoring the ECT for any intermittent jumps high or low. If there are replace the ECT.

b) If the saved info indicates that the engine coolant reading was at the warmest exreme (around 250+ deg.F) then that's a good indication the ECT resistance was intermittently low. Check for a short to ground on the signal circuit and repair as necessary. If it appears okay, warm the engine up while monitoring the ECT for any intermittent jumps high or low. If there are replace the ECT.

Other ECT sensor and circuit related DTCs: P0115, P0117, P0118, P0119, P0125, P0128
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P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
Verify coolant strength & level
Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
If the above items check out good, replace the thermostat
If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code



These codes are telling you that the engine temperature is not sufficient. This can be caused by a faulty coolant sensor,air in the cooling system,or a faulty computer.
The engine should run at 190 F & higher when warmed up. Scan the computer system to view engine temperature.

Keep us updated.

Oct 01, 2011 | 2006 GMC Sierra

1 Answer

Temp gauge says hot(goes all the way to left by gas gauge)little bit of coolant under car once in a while but then coolant will be low-There is no white smoke.Doesn't heat or cool well.heater core has...


the coolant could be just overflow,, as far as the temp,, replace the thermostat as a first thing, when your car gets that hot, it causes readings to be sent to the computer that sends mixed signals to other parts of the car, The first thing you should do is pull the diagnostic codes. This will tell you if the thermostat is dead.check the temp sending unit, it is located under the upper radiator hose, 19mm wrench will take it off, it costs about 20 bucks but you also need to replace the thermostat. all in all its about 25 bucks total and all your gauges work fine....

Jun 17, 2011 | 2005 Chevrolet Impala

1 Answer

Got codes from auto zone po128 &po506


P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
Low engine coolant level
Leaking or stuck open thermostat
Faulty cooling fan (running too much)
Faulty coolant temperature (ECT) sensor
Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
Verify coolant strength & level
Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
If the above items check out good, replace the thermostat
If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code


P0506 - Idle Air Control (IAC) System RPM Lower Than Expected

This P0506 code is one that's sometimes triggered on vehicles that have electronic throttle control. That is, they don't have a regular throttle cable from the accelerator pedal to the engine. They rely on sensors and electronics to control the throttle. In this case, the P0506 DTC (diagnostic trouble code) is triggered when the PCM (powertrain control module) detects an engine idle speed that is lower than the desired (pre-programmed) RPM.

Potential Symptoms: Most likely the only thing you'll notice is the idle speed is lower than normal so it will likely be rougher. There are potentially other symptoms as well. Of course, when trouble codes are set, the MIL (malfunction indicator lamp, a.k.a. the check engine light) illuminates.

Causes: A P0506 DTC trouble code may be caused by one or more of the following:
A vacuum leak
An air restriction in the intake air path or exhuast
A faulty positive crankcase ventilation (PCV) valve
Damaged/failed/dirty throttle body
Internal engine problem
Failed PCM

Possible Solutions: This DTC is more of an informational code, so if there are any other codes set, diagnose them first. If there are no other codes, inspect for vacuum leaks, restrictions, and damage. If there are no symptoms other than the DTC itself, just clear the code and see whether it returns.


Check it, and keep us updated.

Apr 20, 2011 | 2004 Ford F150

2 Answers

P0128 coolant temp below thermostat/regulating temp


P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

Symptoms: You will likely not notice any drivability problems other than the MIL illumination.

Causes: A code P0128 may mean that one or more of the following has happened:
* Low engine coolant level
* Leaking or stuck open thermostat
* Faulty cooling fan (running too much)
* Faulty coolant temperature (ECT) sensor
* Faulty intake air temperature (IAT) sensor

Possible Solutions: Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for Technical Service Bulletins (TSBs), as the ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember to rate and comment this).

Sep 27, 2010 | 2006 Dodge Caravan

1 Answer

The temperature gauge on my 1999 Dodge Ram 1500 is not working. The temperature sensor and the thermostat have been replaced.


Check the gauge by holding in the trip odometer reset button and turn key to run,hold button for 10 seconds this will iniate a diagnostics on all the gauges,and lights if gauge doesn't move during this test replace gauge,if it does move then check the wires on the plug at the engine coolant temp sensor (ects) you can check the sensor by using a ohms meter by unpplugging it and check ohm's or resistance when cold and then start engine as the engine warms up the ohm's reading should gradually lower as the sensor is a thermister and as it warms up the resistance will drop.

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I HAVE A 2007 CHEVY COBALT THE CHECK ENGINE LIGHT CAME ON I CHECKED THE COOLANT TEMP GAUGE THE DIGITAL GAUGE READING WAS ONLY SHOWING THREE BROKEN LINES. I CHECKED COOLANT LEVEL WHICH WAS FINE. ENGINE DID...


I had this problem with a 2007 Chevy Cobalt. I changed the coolant sensor and even though the check engine light went out I still had a reading of "---" on the coolant gauge. The problem is most likely one of three things: a low coolant level, a bad coolant sensor, or the thermostat needs to be replaced. The only other possible thing it could be is a bad BCM (Body Control Module) which in that case you would most likely be having other weird problems.

The Thermostat and the sensor are both very easy to get to and replace. Take off the engine cover by removing the oil cap and they are both to the rear of the engine on the driver's side. They are right next to each other.

Hope this helps!

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