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2003 GMC code P0128

2003 GMC Envoy didnt pass smog. Gave a code P0128 Coolant temperature below thermastat rating. Can you tell me what the likely cause is? We are not art all mechanical but if I have to take it to be repaired I dont want to be mislead. Thanks

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PO128 insufficient ambient temperature.your thermostat is either sticking open or you have the wrong temperature rating it should be 195 degrees.

Posted on May 30, 2009


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2003 envoy was fine till today when I started to move it it went to reduced power and shuts off now showing codes P's*5154, p0128 and p0440 what should I check

IT is very important that the systems you check are repaired in the exact order that the codes were given on the scanner, because of the way the systems & sensors in the car are wired to share info with the computer and the devices, and fixing the very first code given on the scanner will often repair all of the code problems, so verify the order of the codes before repairing anything.

I would strongly advise that you first buy a Haynes Repair Manual for your vehicle before you start. They are well worth the 25$, or if you have access to a computer, go to and get a subscription to their data base for your 2003 Envoy. It has detailed instructions and diagrams which you can print out, and is well worth the cost. Here are the things you need to look for, but do NOT start throwing parts at the car & hoping it fixes the problem. Check for all of the obvious things first, like a loose or unplugged or broken connector or wire, before assuming the part is bad. Check your battery voltage and grounding connections too. Some of the more modern vehicles can give error codes or have part malfunctions with as little as 1/2 to 1 volt drop in voltage!

P1514 is
"Manufacturer Controlled Vehicle Speed, Idle Control And Auxiliary Inputs" which would mean a problem with Idle Control or Idle Air Control which is mounted on the throttle body, and/or any other controls wired into this part of the system.This link can show you all the possible parts involved:*14932%7CL2*14985&sortBy=5&categoryId=14985&fromCategory=yes&pageId=ajaxPartList&category=&keywords=fuel-injection-system&l1_categoryId=14985&l2_categoryId=&partType=&beginIndex=0&filterValue=vehicleId1%3A839767*exact&prCnt=866&pageSize=50&filters=&selectedPartNumber=&qty=1&ship2HomeClicked=

P0128 is Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature) this one is pretty self explanitory, the water in your engine is not getting hot enough at the time of the reading and sets the code, probably a bad coolant temp sensor or wire.
19970220_bwd_wt5122_pri_thmb.jpg BWD Engine Coolant Temperature Sender
  • Exact Fit for your 2003 GMC Envoy XL SLE

P0440 is the "Evaporative Emission System", which is a canister and a couple of valves and hoses someplace under the vehicle, which start at the engine and connect to the fuel tank, and monitors fuel fumes from the tank and controls valves that reburn these fumes or purge them as necessary. This link shows related parts of the evap system:

Sep 26, 2014 | 2003 GMC Envoy

1 Answer

I have a p0128 code on my 2005 gmc envoy xl 4.2 l

Code 128 means the coolant temp is below normal range when the engine should be warmed up. Most common cause is a faulty thermostat.

Jan 29, 2014 | 1997 Plymouth Voyager

4 Answers

Po128 is the code. What's the problem

Well, I can't give you any specific information regarding your vehicle because you forgot to mention what kind of vehicle you have. (This is kinda important if you really want some help figuring out what is wrong with your car)

The "Generic OBD" of code P0128 is "Coolant Temperature Below Thermostat Regulating Temperature"

The most likely cause is a stuck-open thermostat, however, I have also seen defective Engine Coolant Temperature (ECT) sensors also cause this code.

Jun 13, 2011 | Chevrolet TrailBlazer Cars & Trucks

1 Answer

2003 GMC Envoy Service Engine light on. OBD2 Code is P0128

Ah the famous P0128.....It's 1 of 2 things. If your fan is not engaged constantly (would sound like an airplane or big rig everytime you step on the gas) and is working properly, that rules out the fan clutch. The other common problem is the thermostat. Replace the thermostat and the code will go away. Engine temperature needs to be at 210 degrees on these engines. If the fan clutch is constantly engaged, it's cooling the engine off constantly and won't get up to 210 fast enough. If it's the thermostat, it's stuck open and isn't allowing the coolant to properly heat up in time.

Feb 08, 2011 | 2003 GMC Envoy

1 Answer

Hi, I have a 2003 Ford Expedition. The code reader gave a code of P0128. It pop up every once in a while especially in raint weather. How do I fix the problem?

it's for coolant below regualted temperature. You have a cylinder head temp sesnor and ECT, engine coolant temp senor. May be a bad thermostat.

Jan 21, 2011 | 2006 Ford Fusion

1 Answer


It's a thermostat/coolant issue. The definition is here:

P0128 Engine Coolant Temperature Below Thermostat Regulating Temperature

Possible Causes
  • Check the operation of the thermostat (it may be stuck open)
  • Coolant level is too low, or the coolant mixture is incorrect
  • ECT sensor connector is damaged (check pins for damage, and for moisture)
  • ECT sensor signal circuit has a high resistance condition
  • ECT sensor has failed

Jan 20, 2011 | 2003 GMC Envoy

1 Answer

2004 GMC Envoy.Check engine soon light is on the code for the trouble I am having is P0128. This means bad thermostat-it is not running hot, engine coolant temperature sensor which I had replaced,...

This code indeed does indicate a bad thermostat, but not in a way as overheating but the coolant temperature is not rising fast enough at warm up because the stat is always in the open position.

The rushing air sound you heard was most likely the electronic fan clutch starting to go bad. This is a common problem.

Nov 01, 2010 | 2004 GMC Envoy

1 Answer

Check engine light on 2004 GMC Envoy. Had air intake sensor replaced. Failed inspection diagnostic code came up P0128. I have been told I needed an air pump, a tune -up, injectors cleaned etc. etc., I have...

Code P0128 indicates a faulty thermostat, more than likely it is always in the open position causing the coolant to come up to operating temperature too slowly.
My suggestion is to find a different repair shop.

Oct 31, 2010 | 2004 GMC Envoy

1 Answer


DTC P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

What does that mean?
This means that the engine's PCM detected that the engine has not reached the required temperature level within a specified amount of time after starting the engine. The intent of the P0128 code is to indicate a faulty thermostat. Similar codes: P0125

In determining the engine did not reach a "normal" temperature, it takes into account the length of time the vehicle has been running, the intake air temperature (IAT) sensor reading, the engine coolant temperature (ECT) sensor reading, and the speed of the vehicle.

You will likely not notice any drivability problems other than the MIL illumination.

A code P0128 may mean that one or more of the following has happened:
* Low engine coolant level
* Leaking or stuck open thermostat
* Faulty cooling fan (running too much)
* Faulty coolant temperature (ECT) sensor
* Faulty intake air temperature (IAT) sensor

Possible Solutions:
Past experience indicates that the most likely solution is to replace the thermostat. However here are some suggestions on troubleshooting and repairing a P0128 OBD-II code:
* Verify coolant strength & level
* Verify proper cooling fan operation (check if it's running more than it should). Replace if necessary.
* Verify proper engine coolant temperature (ECT) sensor operation, replace if necessary.
* Verify proper intake air temperature (IAT) sensor operation, replace if necessary.
* If the above items check out good, replace the thermostat
* If Nissan vehicle, check for a TSB such as this one. The ECM may need to be reprogrammed to correct the P0128 code

Hope helps (remember rated this).

Jul 01, 2010 | 2004 GMC Envoy

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