P0128 07 Cobalt LT 2.2 - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 4 (permalink) Old 04-30-2018, 09:43 AM Thread Starter
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P0128 07 Cobalt LT 2.2

It's not what you think! Thermostat doesn't seem to be the issue. Let me break it down in the order that the issues happen.

1: Coolant temperature displays "---"
2: Cooling fan turns on
3: Code P0128 comes on
4: Code is read and then Cleared
5: Engine cooling system returns to normal for an hour or up to a week. And the temperature is displayed on the dash again.
6: It starts all over again

Coolant Temp Sensor has been replaced, and there is no corrosion on its connector. The wires look protected in the casing. There is some corrosion on the relay for fan #1, so I cleaned the connection and replaced the relay. Coolant levels are good.

Do I need to replace the ECM? Why is everything working fine when the code is cleared? Check for more corrosion? Car only has 109,000 miles.
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post #2 of 4 (permalink) Old 04-30-2018, 04:10 PM
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Did you replace the thermostat? That code is because the engine doesn't get up to temperature within the specified times. When a thermostat ages it loses it's ability to close tightly and lets coolant by. When this happens the engine temperature is not reached within the specified time because coolant flows around the flap to the radiator and cools the block. All the rest you mention is normal when the thermostat goes bad. When the ECU sets the code it also turns automatically the fans. It's just going to get worse with time.

P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature.

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post #3 of 4 (permalink) Old 04-30-2018, 05:40 PM Thread Starter
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The thermostat has not been replaced yet. I will as a last resort, but the thermostat shouldn't have an affect on the coolant temp sensor. If it is not sealing, I should be able to see the "not warm enough" temp before any code is thrown.

The fan is coming on BEFORE the code because the temp sensor is not reading anything.

The car gets up to normal temp 180-210 degrees. As long as the sensor doesn't kick out.

Am I wrong in my thoughts of how the thermostat affects the temp sensor pre-code?
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post #4 of 4 (permalink) Old 04-30-2018, 06:59 PM
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Nothing is instantaneous, the ECM controls all engine issues the way it was programmed. It's the ECM that sends the reading to the DIC, it's the ECM that sends the check engine light command to the instrument panel and it's the ECM that turns on the fan before or after the code comes up. If your thermostat is 11 years old it is long overdue and has lasted longer than most.

Here is what the GM service data base says.
DTC P0128
Circuit Description
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the powertrain control module (PCM) for engine control and as an enabling criteria for some diagnostics.

The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected.

If the PCM detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

DTC Descriptor
This diagnostic procedure supports the following DTC:

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

Conditions for Running the DTC
• DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0117, P0118, P0121, P0122, P0123, P0502, P0503 are not set.

• The startup ECT is less than 65°C (149°F).

• The intake air temperature (IAT) is between -7 and +55°C (+19 and +131°F).

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

• The vehicle is driven more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph).

• The mass air flow (MAF) is between 2-30 g/s, with the average more than 1 g/s.

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

Conditions for Setting the DTC
The PCM detects that:

• The calibrated amount of engine run time has been met

• The calibrated amount of engine air flow has been met

• The calibrated vehicle speed and distance have been met.

• The calibrated ECT of 70°C (158°F) has not been met

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