2001 GTP DTC Error Codes P1404 and P0128

I recently had my codes ran when the SEL came on and received a P0128, P0404, and P1404 codes. My mechanic mentioned that this would be related to the thermostat and EGR valve so I picked up an EGR valve and replaced it. I waited for a couple of drive cycles and the SEL did not go out so I decided to clear the codes and see what happens.

A day had gone by and the light came back on so I ran the codes again to find only the P1404 and P0128 DTC codes this time. The P0128 refers to the thermostat being bad and I can believe that this would be the case so I will look into replacing it this weekend. However, the P1404 code is showing "DTC Definition Not Found". Before I reset the first time, it stated that the P1404 DTC was related to the HVAC system which I in turn believed to be an issue with the EGR valve.

I have 2 questions. First, can the P1404 DTC be related to the P0128 DTC? Also, is it true that there are problems with GM vehicles needing the computers flashed with a new version? I heard there was an update in 2002 that fixed the random P1404 DTC's that people were getting on their vehicles. Seeing that the code definition is not found on the scan tool I am using, I am wondering if there is something that is out of date.

The scanner is also showing the following monitors as "Not Ready"

Catalyst
EVAP System

All other systems are Ready or Not Applicable

BTW here are the specs on my car

2001 Grand Prix GTP 3.8L SC
No Modifications
155,000 miles

Scan Tool Model
Actron CP9550 PocketScan Plus

Any advice is greatly appreicated.

In general, most hand held code scanners will not read anything above P1000 as P1000 and up codes are manufacturer specific.

Code P0128 is thrown to detect a faulty thermostat but it should also be noted that a faulty ECT (engine coolant temp) sensor or faulty IAT (intake air temperature) sensor could also throw this code. If engine coolant fails to reach its target temperature the code is thrown, but as stated the two above sensors could be providing incorrect data to the PCM.

Code P1404 is a manufacture specific code, it is related to EGR operation. If EGR sensor operation is out of its specified voltage range this code will be thrown, excessive carbon build up could be the cause or lack of vacuum.

The "not ready" monitors you mentioned has to do with trips and drive cycles. It takes all 5 monitors to complete 1 trip and are defined by certain driving characteristics. All 5 monitors plus the catalyst monitor must pass before a drive cycle can be completed. The system cannot complete its monitor tests because a prioritized monitor is not passing.

In general, most hand held code scanners will not read anything above P1000 as P1000 and up codes are manufacturer specific.

Code P0128 is thrown to detect a faulty thermostat but it should also be noted that a faulty ECT (engine coolant temp) sensor or faulty IAT (intake air temperature) sensor could also throw this code. If engine coolant fails to reach its target temperature the code is thrown, but as stated the two above sensors could be providing incorrect data to the PCM.

Code P1404 is a manufacture specific code, it is related to EGR operation. If EGR sensor operation is out of its specified voltage range this code will be thrown, excessive carbon build up could be the cause or lack of vacuum.

The "not ready" monitors you mentioned has to do with trips and drive cycles. It takes all 5 monitors to complete 1 trip and are defined by certain driving characteristics. All 5 monitors plus the catalyst monitor must pass before a drive cycle can be completed. The system cannot complete its monitor tests because a prioritized monitor is not passing.
So should I start with the thermostat since it is the cheapest? Also, where is the ECT sensor? I can usually do most of the maintenence and repairs on my car except when it comes to the electrical so I don't want to get too in depth if it is not something that I can easily check or replace.

T-stat would be easy and from the diagram it looks like the ECT is just below the t-stat housing itself.

As suggested above replace both Thermostat and ECT sensor at the same time, it will save the process of having to drain or spilling coolant twice. Both parts aren't expensive and you'll save time.

When you replace the ECT you can use some teflon tape on the threads, just leave some of them uncoated so they make contact with the engine block to provide a good ground to the sensor.

Oscar.

So this past weekend I replaced the ECT sensor and thermostat.

After running the engine and attempting to bleed the system, I was getting no air release and no coolant from the bleeder. I did add coolant to both the radiator until it was full and also the resevoir hoping to pull the collant into the system.

After running the engine for another 15 minutes, the temp peaked at almost the warning level on the temperature sensor. After various attempts to get coolant to go through the system, I decided to change the thermostat back to the original and try again. Now I was running between 190 and 215 on a consistant basis with no issues and the P0128 code was not coming back. I figured that everything seemed to be in order and things were running good.

Well 2 days ago the P1404 code came back but the car was still running pretty good as far as temperature goes so I wasn't too overly concerned. Well today I was out driving and everything started acting up and the temperature would not go above 180 on the gauge. I checked to see if there were any new codes and sure enough the P0128 was back.

I don't think that the thermostat is bad as it was working correctly but how could the sensor be bad after only a couple of days use?

Is there something I am missing?

If you used aftermarket parts, especially Duralast, both the t-stat and sensor could be bad. The new design AC-Delco thermostat has an air-bleed valve to help get the air out, most aftermarket ones do not. Also, many aftermarket t-stats (Stant and Duralast are two such types) will lock open if the coolant gets into the danger range. I have replaced a bunch of these style for this reason. This is most likely why your guage is reading low and the P0128 code is back.

As far as the EGR code, P1404 is EGR Solenoid 2 error. Since changing the EGR didn't keep the code away, there may be an issue with the PCM, or the wiring between it and the EGR valve.

On the CTS, a quality replacement part will have pre-applied electrically conductive thread sealer on the threads. Using teflon tape or liquid telfon sealer will most likely interfere with the sensor getting a good ground.

Just wanted to thank everyone for their suggestions and help. Here is everything I have had done and the problem is gone.

Replaced EGR Valve (Didn't Fix Either Problem)
Replaced Thermostat (Fixed P0128 Code)

Decided to take my car to my mechanic to run some tests on the computer and he was able to determine a few things. First is that my temp guage on the dash is off on the actual temperature of the engine. When the engine temp reaches 195, the dash is reading 215. When the engine temp reaches 215 and the fans kick on, the dash guage reads almost 240. He said this should not be a problem as the thermostat and fans are working correctly and it was more to give me piece of mind that I was not overheating. He also tested the EGR valve and everything is functioning as well as he was able to open and close the valve and get desired results. I had the cooling system flushed out as well since it had been quite a while and I needed some new coolant.

The only way that I was able to fix the P1404 error was to take it to a GM Service Center and have the PCM flashed with the more current software. When I heard that this was what I needed to do I figured it was going to be a couple hundred dollars. I was happy to hear that it was only $99.50 and took about 35 minutes to do. I went and got it done and now everything seems to be working correctly. The dash guages still show the temp too high for the actual engine temp but since I know this, I am not too concerned.

Thanks again to everyone that gave me their suggestions as I am finally fixed and the car is back to its old self again.

If you are getting an incorrect reading to the gauge, then the temperature sensor itself is bad. There should be two sensors. One to tell the fans to kick on, and one to give a reading to the gauge. The fans could care less what the temperature says on the gauges because it goes off the second sensors readings. I would bet if you replace the gauge sensor then you will be showing correct temps and all will be working as it should.

If you are getting an incorrect reading to the gauge, then the temperature sensor itself is bad. There should be two sensors. One to tell the fans to kick on, and one to give a reading to the gauge.

Actually they are in the same housing, It's just a 3 wire setup. But I agree if you want to get it fixed then replace the ECT as suggested before and be done with it...

Oscar.

Thanks to everyone for their help. Here is what I had to do for each code.

P0128 - Replaced the thermostat and temp sensor and also had the coolant system flushed and everything is working great.

P1404 - PCM flash is the only way that I was able to get this to go away. Ran me about $95.00 at a GM Service Center. I was 3 revisions of the software off from the newest anyway so it was probably a good thing to get done.

P0404 - EGR Valve replacement fixed this.

Now I am getting P0440 which I hope is just the gas cap. Gonna run and pick one up today and reset the SES and hope for the best. If that doesn't work than it is on to the smoke test.

Got my emmissions test by October so I need to figure out something soon. Illinois only does one thing and that is hook up to the computer to make sure there are no pending errors and that all the systems are running correctly.