P0128 diagnostic code

bwenny247 · Post by **bwenny247** » Sun Dec 18, 2005 5:22 am

my truck threw a p0128 DTC the other day coming home from work, not a long trip. i looked it up and found "P0128 - Coolant Thermostat Below Thermostat Regulating Temperature" the general consensus is that the thermostat should be changed.

here's my question: since i have the low temp thermostat (180 degree) and we haven't had a day above 30 degrees in 2 1/2 weeks, do you think that all added up to the "coolent thermostat below regulating temperature"??? i cleared the code, it stayed off, but it came back 2 days later.
what do ya think?
Thanks
Ben

04crewvt · Post by **04crewvt** » Sun Dec 18, 2005 5:58 am

well on my 2004 I just installed the HPPIII and it recommends the stock stat. It could be that the ECM is looking for a certain temp for the coolant in a certain amount of time, and the 180 stat is causing the temp it is looking for to be too low.

JUST FOUND THIS INFO:

generalmotorstech78
10-23-2004, 08:12 PM
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.

This DTC will only run once per ignition cycle within the enabling conditions. 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.

Conditions for Running the DTC
DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P0502, P0503 are not present.
The startup ECT is less than 75°C (167°F).
The intake air temperature (IAT) is more than -7°C (+19°F).
The engine run time is between 30-1,800 seconds.
The vehicle is driven more than 0.8 kilometers (0.5 miles) at more than 8 km/h (5 mph).
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 80°C (176°F) has not been met.

IN OTHER WORDS, BAD T-STAT OR ENGINE COOLANT TEMP SENSOR

This come from 3800pro.com web site so the numbers are based on a 180 normal stat that was replaced with a 160 so substituting our 190 for a 180 probably will have the same effect

Post by **HenryJ** » Sun Dec 18, 2005 6:39 am

What year is your truck?

I think it is time to relocate your IAT sensor. relocating IAT This has been working for me.

You can see where mine is located in this picture-

I made my extender:

HenryJ wrote:I grabbed an eighteen inch section of the IAT wiring harness with the correct plug that attaches to the sensor, and two used sensors , and the grommet from the salvage yard.
I then carefully cut the sensor end off of one of them and soldered the cut end of the wire harness to them heat shrink insulated them and covered the now male on one end female on the other end harness extender that I now have with some black corrugated loom.

Less than $5 worth of parts.

bwenny247 · Post by **bwenny247** » Sun Dec 18, 2005 11:53 am

so by moving the IAT from K&N's position (red arrow) to the new position (blue arrow) this will take care of the SES light?

long term, what kind of damage am i doing by clearing the code and waiting to do this for a while? I'm real busy with work and don't have time to get to the salvage yard. would it be better for me to just buy a replacement stock thermostat until i can get time for this? my soddering skills aren't up to par......actually i have none

i still think the cold weather has something to do with it.....I've had this set-up for half a year without a problem and now when it gets cold this started. hmmmm, i dunno.

thanks for your help

killian96ss · Post by **killian96ss** » Sun Dec 18, 2005 1:05 pm

bwenny247 wrote:long term, what kind of damage am i doing by clearing the code and waiting to do this for a while?

I don't think any damage will result from leaving everything the way it is now. HJ will correct me if am I wrong.

Your PCM is just alerting you to the fact that the temperature threshold isn't being met. If you don't like soldering, posi-lock connectors are very reliable and better than crimp connectors. Soldering really isn't that hard even if you have never done it before. A few practice attempts on some scrap wires will teach you pretty quick. I would at least try the IAT relocation like HJ did before you just go back to a stock t-stat. It worked for him, and he had a similar problem with the SES light.

Post by **HenryJ** » Sun Dec 18, 2005 2:56 pm

bwenny247 wrote:so by moving the IAT from K&N's position (red arrow) to the new position (blue arrow) this will take care of the SES light?

Yes. This lowers the IAT reading by 20 degrees and allows a little more time to reach the threshold.

long term, what kind of damage am i doing by clearing the code and waiting to do this for a while? I'm real busy with work and don't have time to get to the salvage yard. would it be better for me to just buy a replacement stock thermostat until i can get time for this? my soddering skills aren't up to par......actually i have none

I can't say for sure if the engine will richen or lean the mixture to try and compensate. I tend to agree with killian96ss
, in that this is just alerting you of the stuck thermostat. I cleared mine several times in the quest for a solution and although I did suffer a plugged precat, it was probably related to other mods that I had tested.
Somewhere there is an extension kit you can buy. I'll have to look for it.

SONOMA915 · Post by **SONOMA915** » Sun Dec 18, 2005 3:08 pm

There is a harness kit made by Jet, shown here at the bottom of the link. Not sure of the part number. Found it in another post.Harness Just thought this would help on your quest.

Post by **HenryJ** » Sun Dec 18, 2005 3:26 pm

That's the one

Thanks!

96-00 IAT Relocator-96-2000 GM C/K Truck-4.3/ 5.0/ 5.7 61620
Purchase Part #61620

JET Inlet Air Temperature Relocator

Price: $49.95

Post by **Walt** » Sun Dec 18, 2005 11:00 pm

Wow.....it's gone up $10

bwenny247 · Post by **bwenny247** » Mon Dec 19, 2005 4:18 pm

well i bought a replacement 195 degree stat. i think i'll run this one in the winter, or at least until i get time to relocate the IAT

installed the stat sunday 20 minutes (would have been quicker had it been warmer than 15 degrees out and i have to work outside....brrrrrr). got out the hypertech and cleared the code. started it up and brought it up to temp, no light. took a short drive, everything seemed fine.

then i took a short winter wheelin' trip through 12 inches of fresh snow,down into our field

got in a fight with a pine tree....but the crew won

got back into the driveway, checked the fluid level, added coolant.

saw that my K&N needed some TLC so i took that out and cleaned and re-oiled it.

been about 7 or 8 starts now and no light....yeah!!!!!!
i think i'll leave the 195 stat for the winter and put the 180 stat back in for the summer.
I'll keep you all posted
thanks!
ben

killian96ss · Post by **killian96ss** » Mon Dec 19, 2005 4:55 pm

wamason wrote:Wow.....it's gone up $10

They are still $39 at Summit (part # JET-61620) and Jegs (561-61620). The GM IAT sensor part # 12160244 alone costs $15-$20, so your only paying $20-$25 for the nice harness since the kit inludes a new IAT sensor.

Steve

barch97 · Post by **barch97** » Sat Apr 07, 2007 6:22 am

So, is there an actual "fix" for this? It seems you guys have kinda worked around the problem and disguised it. I've still got the stock t-stat and just got the same code.

This seems a bit excessive and I'm WAY out of warranty.

Eric H wrote:Mine came on when I had about 32,000 miles on my truck. It was fixed under warranty. Here's what the write-up said:

"Scope and scan system DTC P0128 ECT ERROR. DTC P0442 Evap
small leak. DTC 90410 Air System. Tested and found the
T-Stat sticking open. Tested and found the fuel cap not
sealing. Tested and found the air pump seized due to the
check valves were blown.
Replaced the T-Stat. Replaced the fuel cap. Replaced the
right and left air pump check valves. Replaced the air
pump and fuse. Cleared DTC's Rechecked OK."

-Eric H

sounds like they replaced a part at a time and checked to see if that was the one then moved on to the next. what's the fuel cap got to do with coolant temp?

is this a "problem" I should be concerned about or just clear the code and wait for warm weather?

F9K9 · Post by **F9K9** » Sat Apr 07, 2007 6:31 am

Clearing it and seeing what happens shouldn't hurt.

barch97 · Post by **barch97** » Sat Apr 07, 2007 6:34 am

Yeah, that's my plan for now.

Is five years and 100k miles a lot for a thermostat? might be worth a few bucks to go ahead and replace it anyway.

Post by **HenryJ** » Sat Apr 07, 2007 6:49 am

barch97 wrote:So, is there an actual "fix" for this?

I am a little lost here for the train of thought. If you are referring to the PO128 code set by a colder thermostat, then yes.
Relocating the IAT will buy enough time to correct the condition. Re flashing the PCM to set the threshold may work as well if there is an option to do so. EFI live or another programming software would be needed.

To correct a stock vehicle with this problem, you need to find the part that is failing.

It seems you guys have kinda worked around the problem and disguised it. I've still got the stock t-stat and just got the same code.

You may have a problem. If your stock stat is setting the code it needs to be checked and perhaps replaced. Start there. That is the easy and cheap part to check. Next would be to check the ECT sensor. If that is good then move to the IAT sensor.
I listed them this way in order of cost and failure rates. The IAT is not as likely as the thermostat.

...what's the fuel cap got to do with coolant temp?

Two different codes and problems.

is this a "problem" I should be concerned about or just clear the code and wait for warm weather?

I always clear the code and see if it reoccurs. Then you know there is indeed something happening. That is unless the problem is very obvious.

roadrunner · Post by **roadrunner** » Fri Oct 24, 2008 6:36 pm

An old thread but relevant. I got this code today (P0128). A quick check of the gauges I was monitoring with my scangauge II revealed the water temp was only 167F. Thermostat is original and 97k miles. Due to replace anyway. Interesting development though. I monitored the gauge activity for the next 3 hours delivering mail and the truck kept shifting back and forth between open and closed loop. The breaking point appeared to be engine temp with 180F as the tipping point. I was monitoring IAT, timing, engine temp, and transmission temp. Then switched trans temp over to open-closed loop monitor and made this discovery. Perhaps this has been a negative impact on fuel mileage lately??? Will be installing new 195F stat and see what happens. Does make me wonder how others here are able to run significantly cooler thermostats though. Brule??? Any thoughts???

Post by **HenryJ** » Fri Oct 24, 2008 6:48 pm

The threshold is 178 degrees. After installing the larger radiator the larger reservoir of coolant was just enough to set the MIL for me.
the PCM uses readings from the ECT and IAT in the predetermined formula to predict the time at which the target temperature threshold should be met. If this does not occur the MIL illuminates.
I moved my IAT up stream and bought enough time to run the larger radiator and a 180 stat.
I actually get better readings at that point. They match the outside temperature much closer.

180 is as cold as you can run. Even then you may run into the situation that I did. There is always a way. Mine runs right at 184 most of the time. I like being a little farther from the edge. this works great for all seasons. Well at least it does here.

roadrunner · Post by **roadrunner** » Fri Oct 24, 2008 6:54 pm

So, what do you think about the dropping in and out on the loop modes Brule?

Post by **HenryJ** » Fri Oct 24, 2008 7:02 pm

That will hurt mileage for sure. Closed loop is an educated guess at what the conditions should be. By no means as accurate as running on real sensor readings.

roadrunner · Post by **roadrunner** » Fri Oct 24, 2008 7:08 pm

A little off topic but relevant to the mileage issue. Do you know or have access to the normal advance curve for our 4.3 W engine?

Post by **HenryJ** » Fri Oct 24, 2008 7:29 pm

Controlled by the PCM. I have Autotap, but have never really wondered about that.

roadrunner · Post by **roadrunner** » Fri Oct 24, 2008 7:49 pm

Yeah, I knew the PCM controlled it but was wondering if perhaps you knew what maximum advance might be or what curve GM might have used. I have observed advance all the way from 0* to 42* but almost seems to run contrary to TPS percentage of opening. Never pings and I know the knock sensor comes into play but there's never a hint it might try to ping either.