P0128 = MPG Tanked !

[Shaw520]

Yep she threw the 0128 code and man did the the mpg's go to hell,... I plugged the torque app in and coolant temps only reaching 176*F,.. although dash gauge says one tick down from 210*F, which would roughly equate to 190*F,.. but the torque app seems more accurate.
SO;,.......... Chinese A/C Delco parts .... Just so frigging cold out for this 57yr old fart to be out there,...but it MUST be done.

 

[mrrsm]

Keep in mind that if you have not changed out your Thermostat recently... if it has gone sideways on you and stays open too long or is stuck in a constant flow position.... even a partial one... Changing out the Temperature Sensor won't solve the problem. With the ambient Cold Weather adding to the mix and bleeding off Radiator and Engine Heat so easily... it might seem that the WT Thermocouple has gone bad... but be a little suspicious of the TS, too.

JFTHOI... Lightly (and I do mean LIGHTLY...) Tap on the Thermostat Housing with a Small Plastic Mallet while the Engine is Idling and heating up and see if the internal spring and poppet valve of the Thermostat loosens up and then goes into detente. You might have to try this (LIGHTLY) a few times to observe whether or not the Water Temperature will start to climb... but if it responds... then the TS will be the culprit. But even if the TS fails to respond to the Light Taps... it does not mean that the Thermostat is not bad. Remember to Clear the Code(s) prior to attempting this unorthodox test.

 

[Shaw520]

Guessing you didnt see the box w thermost,.. Ill be changing both,.. The sensor is precautioary while im there.

 

[mrrsm]

Thanks for the unambiguous clarification... I can't really make out much in the image other than the sensor in the fore ground... and it was the idea of you having to make two trips out there under your present frigid conditions that prompted my first response. Its ALL Good Now...

 

[Shaw520]

Thank You for looking out MRRSM,.. but at 19*f with light snow falling,...a true companion would offer to come and install,... ,...,....

 

[Shaw520]

Done, t-stat and sensor replaced,.. temps running 202-212*f,...no more 0128 code. Bottom bolt on the alternator is definitely a beast, If you bend the A/C line just a bit to the drivers side its easier to access. I took the battery out too, easy enough to do and gives more work room.

 

[paul2005tb]

I have the same code P0128, but I only planned on replacing the thermostat. what made you think that the temp sensor was also bad ?

 

[Mooseman]

It wasn't necessarily bad but because it's in close proximity of the tstat, it should be changed at the same time before it eventually fails.

 

[paul2005tb]

It wasn't necessarily bad but because it's in close proximity of the tstat, it should be changed at the same time before it eventually fails.

Great advice, In the last few days I have observed my engine carefully, trying to make a full diagnostic assessment of my P0128. I notice that my fan starts (fan clutch engages) shortly after then engine starts, even before the engine warms up. This fan should never kick on if the problem were just my thermostat.

I suspect that I have the temperature sensor to replace. I just ordered it and will replace it with the thermostat as soon as I get a break in the weather. Im not certain if that is the full extent of my P0128 problem but it is a start.

What other part could fail to cause the temperature control system to mess up. What is odd is that my temperature dial on my cluster never goes over 190 on the dial, but the fan will kick on at 160. Strange. Why the PCM would send a low temp signal to the cluster and yet at the same time kick on the fan to cool the engine is a marvelous mystery of GM programing.

 

[Shaw520]

The temp sensor's failure rate on these truck is high,.. thats why its always a good idea to change both, virtually the same procedure, (remove alternator ect). Also the clutch fans fail as well, but it sounds like your is working correctly. When my thermostat started to go it through the PO128 code as well,...but the gauge seemed to hover at the 200 range not indicating a problem, but when I hooked up my Torque Pro to the PCM the actual temp was 179*,... I believe the threshold for the PO128 is 183 degrees.

 

[xavierny25]

The temp sensor's failure rate on these truck is high,.

Really???? Where did you read this??? Or get that info. I've got 170k miles on my 08 Voy with the oem sensor and no sign of failure. I did go through 2 thermostats 1 was the oem and the 2nd being a stant. I got an acdelco the last time around and so far so good. Don't get me wrong I do have a new temp sensor in my bag of goodies for the truck but I have found no reason to replace it since the original has been reading right all its life.

 

[Matt]

I have to agree with @xavierny25 I have the original temp sensor on my 02 with 165XXX miles but have replaced the thermostat twice as well.

 

[paul2005tb]

OK, if not the temp sensor then is it possible that in addition to the thermostat a failing radiator cap that is lower than 15psi is causing my cold engine problems.

The reason I suspect there is more that has failed than just the thermostat is because my fan kicks on when the engine is still cold. The thermostat is open more than is should be but the PCM is telling my fan clutch to kick on.

 

[Blckshdw]

my fan kicks on when the engine is still cold.

If you do the rag test, and stop the fan along the edge of the plastic rim (not the blades directly), do you feel the PWM pulse of the clutch engaging and disengaging repeatedly, or no?

Edit: My fan always spins when the engine is on too, but I believe that's normal behavior and it's relatively silent. If it was jet engine loud, like on initial cold startup before settling down, then I'd think something was wrong..

 

[Chickenhawk]

The problem with the coolant sensor is not necessarily that they have a high failure rate. It is simply that the engine gets ALL its temp input from that one sensor. So if the PCM THINKS that the coolant temp is too low (because of a bad sensor), or it actually IS too low (because of a bad thermostat) there is no way to tell for sure. Generally, if the temp has been low for a while, it is thermostat. If it only recently dips low and dips up and down from above the center tick to below the center tick, especially at highway speeds, it is usually the sensor.

But the reality is that there is no way to tell for certain. The sensor lives a hard life submersed in coolant its whole life.

The ironic part is that the gauge is NOT linear in reading, but we have found with almost 100% certainty that if the needle is slightly below the center tick, even by a small amount, you will NEED to change one (or both.) This is why we often tell people to just take a photo of their gauge and post it. Measuring temp with a scan tool is useless in trying to decide if its thermostat or sensor because the scan tool gets its input from the same sensor.

So this is why there is a high failure rate. They might not always fail a lot, but the combination of both fails a lot on our platforms.

 

[xavierny25]

Be it as it may, I beg to differ..... I have always been under the assumption that we only advices ppl to change the temp sensor on 2002-2005 models because it was in the general area of a thermostat replacement. And it being a fairly cheap, more of a "fail safe" if in chance the sensor isn't or wasn't reading correctly..... which is still not 100% proven as a high failure part.
Edit: but more of a "I replaced it to be on the safe side and it can't hurt I'm already here"

 

[Mooseman]

I'm thinking it's the fan clutch. It's normal for it to roar after startup and then quiet down. It always spins on friction. Look for my PWM electro-viscous fan clutch test thread. If it's constantly pulling on a relatively cold or just warm engine, try pulling the fan relay and see if it still pulls. That would mean the clutch is going.

 

[Chickenhawk]

Two times I have had my coolant fall slightly below center. The indicator is not linear, which is why we say, if it is even slightly left of center, you have a problem. The first time I suspected it was the sensor because it came on quickly and it would wander up and down on the highway. I took a chance and replaced only the sensor so that I could better report back to this forum.

I was right. It lasted six years with no further problems.

The second time was recently, and it fell below center and didn't waver. I suspected the thermostat, but because I had a good idea what a failing sensor looked like already, I replaced both.

So out of two times, once was sensor for sure, and one was likely one or both. So in my case, sensor failure was at least 50% of my occurrences.

The reason we always advise to replace both is not that they are side by side. It is because there is no way to tell which it is with 100% certainty, and the consequences of not fixing it quickly are expensive and severe. If either the sensor fails or the thermostat fails, the PCM pumps in raw fuel thinking the engine is running cold. This results in a cat converter that is going to clog in relatively short order. We have had LOTS of members having to replace cat converters at some point in their lives ... and every single one reported that at some point in the past, the engine temp was below specs. It does not even need to be low enough to trigger a code; driving long term with that gauge one tick below center WILL cook that cat sooner or later.

I fix mine right away. I test the performance of my vehicle every now and then, and the highway acceleration (the most telling indicator of cat failure) has not dropped since the day I bought it 12 years ago.

THIS is why we advise changing both.

 

[Mooseman]

What he said. However, the reason I'm also suspecting the clutch is because IF the sensor is reporting a low temp, the fan should not be engaging.

 

[Chickenhawk]

I completely agree about the fan clutch. I was simply addressing a really good question on why we recommend replacing coolant temp sensor with the thermostat.

I have been fortunate to have never needed a fan clutch. Knock on head.

 

[paul2005tb]

1) So grateful for you guys. I have my new AC Delco thermostat and temp sensor and will install as soon as the weather lets me. Regarding the thermostat I notice that the actual thermostat element is very simple and can be easily removed from the housing that it comes with. Not sure why AC-Delco chooses to sell the entire flange and pipe and thermostat as one unit. If I can purchase the thermostat separately I could put it into the existing flange assembly.

2) As for the fan clutch, I will have to put that off for now but will gather the will to put a cloth rag to the nylon blades and see if I feel the pulses. If I dont, ill know it is the clutch itself that is sticking. I really wanted to go to a fully electronic fan with its own temperature sensor anyways.

3) Finally performed the requisite test of the fan clutch. I grabbed the fan with a rag and it stopped effortlessly and I felt no pulsing force. May have to rebuild my fan clutch, might be time to go completely electric.

4) If you have taken a look at the temperature sensor location and tried to think of a way to get an 18mm wrench around it you know it is a puzzle. The same problem will be faced when putting the new one on. I could easily cut the wires on the old one but would still faced with the cramped location and torquing the new one.

Since I can not find an 18mm slotted socket on the market, I will buy a 22mmm slotted socket (they are everywhere and are very cheap due to the 22mm standard for O2 sensors), and I will grind a 22mm nut to have an 18mm inside hex shape. I will slot this nut and this will be my adapter for my 18mm temperature sensor.

 

[paul2005tb]

I finished the job yesterday, what a difference, the engine came up to temperature quickly and was locked dead center on the 210 mark. Looking forward to getting back that lost 2mpgs. This job was a bear.

By the way, I always wonder when to put anti-seize and when to put on medium strength loctite. I decided to put antiseize on the alternator mounting bolts and I put medium strength loctite on the brass temperature sensor threads .