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Discussion Starter #1
Hi folks - this is a friend's car so I'm not gonna be a regular, but registered to try to get this fixed.

It's intermittently setting a P0128 with no AC and the fans on full and the gauge at the bottom as I have seen described in various searches. I replaced the ECT which did not help. I've measured the connection to the ECT through to the multi-pin connector at the left front of the engine. I'm not confident of the connection through this connector, but it isn't setting a P0118 (open circuit) so it's probably OK. I will pick up a pair of those connectors next time I'm at the junkyard, but for not don't want to rick damaging it during disassembly.

I've read that the PCM tries to determine if the thermostat is not closing completely by watching how long it takes for the engine to heat up based on the intake air temperature at start up. If it thinks it takes too long it sets a P0128, and then once that's set the firmware disables the gauge, AC and turns on the fans (i.e. a firmware bug). When the gauge is working the temperature rises promptly and stays right at 200ish degrees as it should - then it drops to the bottom which is clearly not real.

I've read there is a TSB 07-06-02-001B in this thread ( https://www.g6ownersclub.com/forum/12-problems-solutions/5091-over-heating-help.html#post35785 ). I also found 07-06-02-003, but these don't pertain to the 2008.

Does anyone know of a solution to this issue on a 2008? I can pursue the connection at the multi-pin connector, but don't really think that's it. I can replace the thermostat in the hope that a new one will seal a little better when closed, but have read many posts where that did not help. I'd rather the PCM did not go into hysterics over a non-issue.
 

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Discussion Starter #3
Replace the thermostat. That's the number one cause of P0128 code.
Thanks, I'll probably have to try that, although they've made it quite difficult to get at. I've certainly seen worse so I'll just have to deal with it.

I was hoping someone had info on a firmware update to correct the bugs. P0128 is "Coolant Temperature Below Thermostat Regulating Temperature", which is incorrect. The thermostat holds exactly the correct temperature, though it may take a bit too long to get there.

Next, the system is clearly going into an over-temperature emergency mode - shutting off the AC and turning the fans on full - which is simply a coding error.
 

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Ok, you need to make up your mind, does the gauge stay at the cold end or does it eventually get to the correct temp?
Please re-read the original post: "When the gauge is working the temperature rises promptly and stays right at 200ish degrees as it should - then it drops to the bottom which is clearly not real."

The gauge is run by the PCM, it's not an independent temperature gauge.

Once the system sets the P0128 the gauge is disabled and no longer provides any information. If I reset the codes when the engine is warm it works great, holding a temperature within a couple of degrees of the 195 set point. Then the system sets an erroneous P0128 code and the gauge is disabled, along with the AC and the fans are turned on full. It will not operate the gauge or AC again until the code is reset.

Thermostats rarely fail intermittently, they are run by a wax pellet and when the fail they simply stop moving and are stuck at whatever position the spring pushes them to. It cannot regulate perfectly one moment and be fully open the next, only to start working again when I reset the code, It might not be sealing properly when fully closed, allowing a bit of coolant to bypass it and making it warm up slowly, but who really cares and why would you disable everything because of it?

There is some info here: https://www.justanswer.com/pontiac/81lgp-pontiac-g6-code-p0128-coolant-temp-low-gauge-dash.html
 

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Perhaps these details from a diagnostic guide will help. As you already mention there are separate codes for open and short circuits. Could I also suggest that being snarky with someone who is trying to better understand the problem in order to help may not get you the responses you desire :). Hope this helps and good luck with the fix

Circuit/System Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and supplies a ground to the low reference circuit. The purpose of this diagnostic is to analyze the performance of the thermostat by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also meet the calibrated target temperatures under various operating conditions. The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

Conditions for Running the DTC
  • DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118 are not present.
  • The start-up ECT is less than 75°C (167°F) .
  • The IAT parameter is more than -7°C (+19°F) .
  • The engine run time is between 30-1,800 seconds .
  • The vehicle is driven more than 0.8 kilometer (0.5 mile) at more than 8 km/h (5 mph) .
  • This diagnostic runs once per ignition cycle when the above conditions are met.
Conditions for Setting the DTC

  • The control module detects that the actual amount of accumulated air flow is more than the predicted amount of accumulated air flow before the ECT reaches 80°C (176°F) .
Diagnostic Aids

  • DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.
  • Inspect the ECT sensor terminals and the ECT harness connector for corrosion. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.
  • This diagnostic runs in a specific range. Measure and record the resistance of the ECT sensor at various ambient temperatures between -7 to +80°C (+19 to +176°F) , then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance.
  • A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.
Circuit/System Verification

Caution: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

  • Turn OFF the ignition.
  • Inspect the cooling system surge tank for the proper coolant level.
  • If the ignition has been OFF for 8 hours or more , the ECT and the IAT should be within 15°C (27°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
  • Use the scan tool to verify the proper operation of the engine cooling system fans.
  • If the vehicle passes the Circuit/System Verification test, then operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records list.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.

  • Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis. See: Engine, Cooling and Exhaust\Cooling System\Testing and Inspection\Component Tests and General Diagnostics
Circuit/System Testing
  • Ignition OFF, disconnect the harness connector at the ECT sensor.
  • Ignition OFF for 90 seconds , test for less than 5 ohms of resistance between the low reference circuit terminal A and ground.
  • If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
  • Ignition ON, verify the scan tool ECT parameter is less than -39°C (-38°F) .
  • If greater than the specified range, test the signal circuit terminal B for a short to ground. If the circuit tests normal, replace the ECM.
  • Install a 3-amp fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT parameter is greater than 149°C (300°F) .
  • If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  • If all circuits test normal, test the ECT sensor.
Component Testing

  • Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance table.
 

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Discussion Starter #7
Perhaps these details from a diagnostic guide will help. As you already mention there are separate codes for open and short circuits. Could I also suggest that being snarky with someone who is trying to better understand the problem in order to help may not get you the responses you desire :). Hope this helps and good luck with the fix

Circuit/System Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and supplies a ground to the low reference circuit. The purpose of this diagnostic is to analyze the performance of the thermostat by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also meet the calibrated target temperatures under various operating conditions. The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and reach the calibrated target temperatures.

Conditions for Running the DTC
There was no snark in my reply, I merely directed the poster to the part of my post he misunderstood.

Thank you for the description - this is the diagnostic routine I described. It is looking at the rate at which the engine heats, not whether it gets to full temperature, and setting a P0128 if it thinks it took too long. Then it goes into what is clearly an emergency overheat protection mode, which is an error. Nobody really cares if it took a little too long to heat up but GM. There are many reports of this issue on the internet, Including this one from this site describing the exact problem (on a 2.4): https://www.g6ownersclub.com/forum/12-problems-solutions/47345-p0128.html The guy put 6 thermostats in it.

So I was hoping maybe someone knew of a firmware update/TSB that addresses the calibration of the warm up test and the bugs in the way the PCM reacts to the results of the test, which is the actual problem. There's nothing wrong with the thermostat or ECT.
 

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Discussion Starter #8
The news for the owner is that the temp gauge has been working and the CEL is off. Changing the ECT did not fix it as it still set codes and misbehaved after. The last thing I did was to partially disassemble the female side multi-pin connector on the left front and reseat the sockets as best I could, then reset the codes. If it had been truly open circuit then it should have set a P0118, but perhaps it was a resistive connection.

Anyway, if you run into this common problem (and GM's firmware bug), it might be worth a look at that connector.

I'm done here - I provided much more info than I received.
 

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I recently got a P0128 code on my 2008 GT Coupe 3.5L. First clue of an issue, Your Check Engine Light will be illuminated. Second clue of an issue was the temp gauge was at the bottom of the gauge and did not move. It did move when the engine was first turned on though. All the gauges would do their normal sweep including the temp gauge (also a clue that the gauge itself works), then it would drop to the bottom of the gauge. Third clue was both radiator fans were on all the time. There is one item on the engine that tells you the coolant temperature. That is the Engine Coolant Temp Sensor (ECT). The code mentions the thermostat... to my knowledge there are no electrical connections on or in the thermostat. That leads us back to the ECT. I changed mine. It is the sensor in the back side of the drivers side, radiator side cylinder head. It is the two wire sensor under the throttle body. I used a simple 3/4" open end or "box" wrench to remove it. Some coolant will come out. This took 10 min total from start to finish. By finish I mean new part installed and all parts back on the car, key turned, engine running. The check engine light was still there, the gauge did not work.

The reason this did not work is because the car still see's a problem. How? The answer is the code that is still stored in the engine computer. Disconnecting the battery does not remove this code. Hook up a scan tool, read the codes, you should still see the P0128 code, erase the codes. The check engine light should be off. Go drive your car for a while. All should be well at this point.

The ECT sends a signal to the engine computer and tells it when the coolant is up to temperature and when the engine is warm enough to go into "closed loop mode" normal operation.
 

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I recently had a go around with the thermostat in my Honda Prelude. The temperature gauge would fluctuate wildly. Make sure that there is no air in the cooling system. Temperature sensors need to see hot water, not steam.
 

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A faulty thermostat or ECT sensor can set the same code. You need to diagnose further to see which one is actually faulty. In general, thermostat failure is more common. You can confirm that by timing how long it takes for the coolant to warm up to normal temperature under normal "no fault" mode and operation.
 

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Well 7 days later the code was back. I swapped out the replaced part and again, 7 days later, the code is back. Two brand new ECT sensors and I have to suspect as suggested that the thermostat is to blame. So in two days I will replace that too. By way of the passenger headlight opening.
 
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