Mechanics

ENGINE LIGHT- ERROR CODE P0128

2003 Cadillac CTS • 6 cylinder 2WD Automatic • 43,300 miles

2003 Cadillac CTS Error code P0128

I've got a 2003 Cadillac CTS that shows the P0128 error code. I've read other posts but I am still unsure of just what needs to be replaced. I am not a experienced mechanic.

The car has a temp gauge. After the car has run for a short period the radiator cooling fans will come on low even though the temp gauge indicates the engine is only half its normal temp. The engine tends to continue to run just a bit cool though on a long drive the normal temp may be reached momentarily. It seems if the fans did not run all the time the temp may be normal.

I don't believe it is the thermostat, but could it be the ECT sensor? Would it not go bad such that it would read like the engine is cool, not hot? (I can test the sensor with an ohmmeter if I can find it!). I've read about the AIT sensor, could that be it? The weather has been very cold lately.

Any Help is appreciated!

Tom
Avatar
Terans
January 20, 2011.




The IAT sensor would not be a cause for this. I have only replaced thermostats to correct this concern, and I have done it on several engine variants with no reoccurring problems. Basically, the code sets in respect to how long the engine takes to reach operating twemperature, if I am not mistaken, and a poorly regulating thermostat would cause it. I suppose in theory, it could be the coolant temp sensor. If you have access to a scan tool that reads data, check and compare the IAT, and ECT after a cold soak to determine if there are congruent. Watching it heat up, the IAT is only going to get as warm as the incoming air, and the comparison is irrelevant at that point.

Factoryjack
Jan 27, 2011.
Thank You.

I guess I'll just have to resign myself to replacing the thermostat at some point. I'm not experienced with the NorthStar engine (I believe it's the 3.2L) and have only basic tools like socket sets, etc. Is this a job I should even consider? Since I've read replacing the thermostat requires removal of the timing chain it sounds like I might get in over my head.

Tom

Tiny
Terans
Jan 27, 2011.
It is a 3.2, but is not a northstar, this engine is built by the folks that brought you the Opel. It is a bit of a challenge to do, and you are going to need more than just a six point socket set. This engine makes wide use of the external torx type of fastener, if you know what a torx fastener is, an external torx is a reverse model. You do not have to remove the timing belt, this is belt driven, not chain. You may not want to attempt if you have doubts, you could cause more problems than correct. The flat rate time for thermostat replacement is over 3 hours, for your information.

Factoryjack
Jan 27, 2011.
We'll, I'm not sure I should attempt this job, I guess I'll do more research be fore I decide. But I am still just a bit perplexed by the fact the cooling fans come on before the engine gets close to its normal temperature. If the engine runs too cool too long because of the thermostat being bad wouldn't the fans never come on?

Thanks-
Tom

Tiny
Terans
Jan 28, 2011.
Is your a/c compressor on, and/or front defroster being requested? Either of these will override the fans on to pull air across the condenser, especially when the vehicle is not in motion. It may not be the case that the engine is never reaching operating temp, it could be that the thermostat is opening prematurely, adding time to the warm up process. If this wasn't a federal requirement, you may not even suspect you have an issue. The intention is to get the engine as warm as possible, as fast as possible, in an effort to lower the exhaust gas emissions, as fast as possible.

Factoryjack
Jan 28, 2011.
I noticed my wife had the AC set to ON even though it should never come on with the current outside temps. But when I set the AC to OFF the fans stopped coming on early as reported in an earlier post. Before this test I reset the P0128 error code and so far it has not come back on. It does seems the engine still heats a bit slowly but it does get there more quickly than before. It may be wishful thinking but I'm hoping the error does not show up again. What do you think?

Thanks-
Tom

Tiny
Terans
Feb 8, 2011.
The system will still function as though it is coming on, although your compressor won't engage. As far as the cooling fans, I am not sure if they still come on or not. A faulty thermostat doesn't necessarily mean that the engine is not reaching operating temperature, it may lie in how long it takes to reach. I have included the literary portion of the diagnostic trouble code that may explain further.

CIRCUIT DESCRIPTION
An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control, and as an enabling criteria for some diagnostics.

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

If engine coolant fails to reach a present target temperature before a calculated air flow is accumulated DTC P0128 will set.

This DTC is continuously monitored during engine operation.

CONDITIONS FOR RUNNING THE DTC

DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, or P0118 are not set.
The engine block heater has not been detected or delayed for 20 seconds.
The engine off time is more than 15 minutes.
The engine speed is more than 960 RPM.
The engine coolant temperature (ECT) is less than 50 C (122 F) at start up.
The ambient air temperature is more than -39 C (-38 F) and less than 50 C (122 F).
The vehicle speed is more than 15 km/h (9 mph).
The mass air flow (MAF) average reading more than 23 g/s.
CONDITIONS FOR SETTING THE DTC
The time for the engine coolant to reach a set temperature has been exceeded.

ACTION TAKEN WHEN THE DTC SETS

The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
CONDITIONS FOR CLEARING THE MIL/DTC

The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
Clear the MIL and the DTC with a scan tool.
DIAGNOSTIC AIDS

DTC P0128 is designed to detect a faulty thermostat.
A skewed ECT sensor can cause a DTC P0128 to set.
A skewed IAT sensor can cause this DTC to set.
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Factoryjack
Feb 9, 2011.
Very informative and useful reply. I will keep this information handy as I watch the car's operation.

Thank You very Much!

Tom

Tiny
Terans
Feb 9, 2011.

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