2008 3.5 P0128/Temp Gauge at 0/No AC - Pontiac G6 Forum
 
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post #1 of 9 (permalink) Old 07-13-2019, 12:39 PM Thread Starter
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2008 3.5 P0128/Temp Gauge at 0/No AC

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/1...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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post #2 of 9 (permalink) Old 07-14-2019, 01:23 AM
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Replace the thermostat. That's the number one cause of P0128 code.
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post #3 of 9 (permalink) Old 07-14-2019, 11:52 AM Thread Starter
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Originally Posted by mrslcom View Post
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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post #4 of 9 (permalink) Old 07-14-2019, 12:22 PM
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Quote:
Originally Posted by NotMyGM-G6 View Post
... the gauge at the bottom as I have seen described in various searches.
Quote:
Originally Posted by NotMyGM-G6 View Post
The thermostat holds exactly the correct temperature, though it may take a bit too long to get there.

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?
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post #5 of 9 (permalink) Old 07-14-2019, 03:45 PM Thread Starter
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Originally Posted by dxG6 View Post
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/8...auge-dash.html
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post #6 of 9 (permalink) Old 07-15-2019, 04:37 PM
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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 75C (167F) .
  • The IAT parameter is more than -7C (+19F) .
  • 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 80C (176F) .

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 +80C (+19 to +176F) , 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 15C (27F) 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 -39C (-38F) .
  • 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 149C (300F) .
  • 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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post #7 of 9 (permalink) Old 07-15-2019, 05:42 PM Thread Starter
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Quote:
Originally Posted by boomer View Post
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/1...345-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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post #8 of 9 (permalink) Old 07-16-2019, 04:22 PM Thread Starter
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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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post #9 of 9 (permalink) Old 07-16-2019, 08:07 PM
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Originally Posted by NotMyGM-G6 View Post
I'm done here - I provided much more info than I received.
Hahaha, yes, we are grateful for that. The first part.
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