CullinanCut

I have a 2007 Chevy Cobalt LS that has been throwing a p0128 code recently coupled with the fact that when it does the engine temp readout on the DIC reads "---" instead of a temp. I was told that the most likely culprit was the engine temp sensor (coolant sensor) which I had replaced a month ago and it did not help. Car has 47000 miles so warranty is not an option. The problem comes and goes but in the last month the problem has been more and more frequent. Also now the car has a loud noise when accelerating past 3000 rpms, sounded to me at first like a leak in the exhaust somewhere in the engine compartment but now I'm not too sure. Also on startup sounds like I have a loose belt or something. I have been told by a parts guy at a local dealership to replace the thermostat and flush the coolant and refill but when I posted a thread about this before the general concensus seemed to be that the thermostat would have nothing to do with the temp readout in the DIC.

What should I do next? Could I have caused damage to the engine? Any help would be greatly appreciated. I think I either have a few problems going on at once or I am really afraid that this original problem has now caused some engine damage that would cause the new noise. Thanks!

ChevyMgr

Thermostat should fix this. Don't know about the noise.

CullinanCut

For the help!

If the thermostat is not working properly what kind of damage could I has caused? I noticed that whenever my check engine light is on my fan is constantly on so I can't see it overheating but could I have caused any damage to the engine from it not getting up to the right temp?

rukkee

iTrader: (3)

If you have no heat at the vent's or inconsistent heat i would look at the thermostat .

CullinanCut

I have heat just no AC

08G5GT2.4L

P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature ECM, PCM

I doubt its the actual thermostat on the grounds of the "---" temp read out. More then likely the ECT sensor, reads 0 or --- and tripping the code since it has no ect signal. Your cooling fans are also running all the time to right? If it was the thermostat youd know since you would still have a temp read out and your engine would be freakin hot.

The noise is probably something separate and your just noticing it now.

rukkee

iTrader: (3)

This might not help but this is GM's diagnostic instructions for that code. I am NOT a TECH

DTC P0128
Diagnostic Instructions
• Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors
DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Diagnostic Fault Information
Circuit
Short to Ground
High Resistance
Open
Short to Voltage
Signal Performance

ECT Sensor Signal
P0117
P0118
P0118
P0118
P0116, P0128

Low Reference
--
P0118
P0118
P0118
P0128

Typical Scan Tool Data
ECT Sensor Temperature Circuit
Normal Range
Short to Ground
Open
Short to Voltage

ECT Sensor Signal
-39 to +120°C (-38 to +248°F)
150°C (302°F)
-40°C (-40°F)
-40°C (-40°F)

Low Reference
--
--
-40°C (-40°F)
-40°C (-40°F)

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, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0121, P0122, P0123, P0502, P0503 are not set.

• The start-up IAT is between -7 to +55°C (+19 to +131°F).

• The start-up ECT is less than 50°C (122°F) when the start-up IAT is less than 10°C (50°F).

OR

• The start-up ECT is less than 75°C (167°F) when the start-up IAT is greater than 10°C (50°F).

• The engine run time is between 30 seconds and 30 minutes.

• The vehicle has traveled greater than 0.8 kilometers (0.5 miles) at greater than 8 km/h (5 mph).

• The mass air flow (MAF) average is greater than 1 g/s.

• This DTC runs once per ignition cycle within the enabling conditions.

Conditions for Setting the DTC
• The ECM detects that the minimum ECT of 55°C (131°F) has not been met with a start-up IAT of less than 10°C (50°F).

• The ECM detects that the minimum ECT of 80°C (176°F) has not been met with a start-up IAT of greater than 10°C (50°F).

Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.

Conditions for Clearing the MIL/DTC
DTC P0128 is a Type B DTC.

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.

Reference Information
Schematic Reference
Engine Controls Schematics

Connector End View Reference

• Engine Controls Connector End Views

• Engine Control Module Connector End Views


Electrical Information Reference

• Circuit Testing

• Connector Repairs

• Testing for Intermittent Conditions and Poor Connections

• Wiring Repairs


DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

• Scan Tool Data List

• Scan Tool Output Controls

• Scan Tool Snapshot Procedure


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 engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
•If the ignition has been OFF for 8 hours or greater , 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.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.

•Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis .
Circuit/System Testing
Important: All electrical components and accessories must be turned OFF and allowed to power down.

•Ignition OFF, disconnect the ECT harness connector at the ECT sensor.
•Ignition OFF, 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 Sensor 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 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT Sensor 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 the 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. Refer to Temperature Versus Resistance .

Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• Engine Coolant Temperature Sensor Replacement

• Engine Coolant Thermostat Replacement

• Control Module References for ECM replacement, setup, and programming
Š 2009 General Motors Corporation. All rights reserved.

CullinanCut

So I have already replaced the coolant temp sensor, what would the next step be? I read in another thread that somebody had this problem and it was fixed by getting the ECM reflashed , if I remember correctly. I would really like to fix this myself if I can but I'm begining to think I may have to bite the bullet and pay the diagnostic fees

BULLETSSMOKE

I think that thread may have been mine and it didnt fix it. The CEL comes on about as often as yours but it ONLY comes on in cooler weather. Never above 75 degrees. Usually if I start the car and its 70 degrees or less it will come on and then go away after driving like 50ish miles or so and 3 engine startups. It will also come on if I have been driving in 70 degree or colder weather for at least 30 minutes or something. ALSO my air conditioning doesn't work likes yours. The compressor does not turn on at all. There are no freon leaks but the freon always manages to leave the system and blows only outside air when I turn the snowflake A/C button on. I have been to a Castrol tech to look at this and even my dad's Lexus dealership. This truly is some weird crap and it was definitely a HOT summer.

rukkee

iTrader: (3)

I would double and triple check the connections to the coolant temp sensor , then possibly get a coolant temp from the system to see what its running at . The AC won't come on cause the car doesn't want to add that load to the engine when it thinks it's overheating or not warm enough.....that is my guess anyways.

BULLETSSMOKE

where exactly is this sensor so i can check it out

CullinanCut

Thanks for the replies everybody, I appreciate it

I think I found the coolant temp sensor on the back of the motor to the right side, when standing in front of it. I think I can see it looking down on the motor from above with the hood popped. But where does the wiring go to? Where would I have to trace it to double check that it is connected correctly?

ChevyMgr

I still think it's your thermostat, and you do have warranty. It's called powertrain. 5 years 100,000 miles. At the very least you would have to pay a diagnostic time if it isn't the thermostat. But you would know what is wrong. And you don't seem to have the technical training to figure this out yourself. It is not uncommon for dashes to display with this code, but you could have other problems.

CullinanCut

I have called three dealerships in the area and they all said that the engine temp sensor would not be covered under warranty so I don't know if the thermostat would be. They said that basically the only things the powertrain warranty would cover were internal engine components, things that oil would touch. You're probably right I will just have to take it in and pay the diagnostic fee I guess

ChevyMgr

Temperature sensor isn't covered, but the thermostat is.

CullinanCut

sweet, thanks!

Tim07

I have the same problem with my G5, Have you solved your problem yet

rollingboostedss

I have been have the issue as well but my heat work and my a/c I'm starting think its just because of the 40* weather that we are having

Maven

iTrader: (1)

There is absolutely zero reason that failed thermostat would cause the DIC to shows dashes,(dashes mean the data the DIC is recieving is outside what it considers normal, IE too high or too low) a failed/failing Tstat can cause a P0128 and that is covered under the 5/100 warranty but the the dahses on the DIC arent, unless they are caused by a failed ECM. If the dashes are caused by a bad connection at the ECT sensors its not covered, neither are poor connections at the computer, or corroded or otherwise damaged wiring.(but all these things can still absolutely be the cause of the P0128) Assuming your sensor is good, First thing you can do if you know where the sensor is, is to disconnect (with key OFF)and look at the terminal in the harness side of the connector, they should be clean and dry, no signs of corrosion and no dark marks, the temrinals should appear undamaged and the female portion should still look "tight", there should be a curved portion of the terminal leading into the female opening, if the opening has no curved area and just appeares flat and square, its damaged. If it looks okay get yourself a little electrical contact cleaner and spray the connection down, let it dry (a few seconds) and then ideally you should put some dielectric grease on the terminals and carefully reconnect it and disconnect about 3-5 times, make sure it still has plenty of grease on it and then reconnect it. If this doesnt fix the issue, its going to be actual wiring, the connection at the ECM (which you can do the same excercising of the coneection to ,be VERY careful, and dont use too much grease, youll hydrolock the connector and break it) or the ECM itself.

This of course assumes that your coolant level is correct and there is no air in the system and that your ECT sensor is in fact good since it was replaced.

hizzo3

2.4l '06 Also have a p0128 code throwing... Replaced the engine coolant temp sensor, no fix... It seems to randomly throw the dreaded "---" on the DIC. I also have some sort of intermittent MAF/MAP mismatch that is causing my T/C to switch off and tranny switch shift times to 0 delay between shifts... Starting to think its the ECM. Anyone have a test procedure for testing the ECM &/Or the wiring harness leading up to the ECM for shorts?

Took it to the stealership for the prior MAF/MAP mismatch, guy drove it, couldnt figure it out and replaced the MAP and said it was fixed... total fail....I figure I have better luck myself. Wonder if HPT with the datalogger would help shed light on the whole issue.

BULLETSSMOKE

im starting to wonder now if my air conditioning problem has something to do with my "---" problem. because the "---" happens below 60 degrees and will stay all dashes until the temperature outside warms up. it stays at a constant --- until it warms up. and my air conditioning is electrical too because the computer doesnt think there is a A/C there but we forced it somehow to work for one day so I know for a fact the compressor is fine. this is some weird ****. I had someone tell me its the BCM but IDK anymore.

Maven

iTrader: (1)

your A/C wont come if the ECM thinks its too cold.

BULLETSSMOKE

wow.

hizzo3

Won't come on if it does not trust the thermostat. My a\c does not work while I have a “---”. When it shows a temp it then works. Oddly I don't have sporadic readings. Its a reading or dashes. Also my fuel gage just started not registering correctly after the last 1/8 tank... Really makes me think its one of the computers.

Maven

iTrader: (1)

the dashes on the display mean the reading is too hot or cold. Doesnt have anthing to do with the Tstat

if your gas gauge is inaccurate its highly possible the sender is damaged by high sulfur content fuel.