Troubleshoot a Code Reading

[Palf70Step]

2002 Silverado
4.3 V6, automatic, about 140,000 miles

Driving to work this morning, just as I pulled in the parking lot My service engine light came on. Drove home, light still on, ran code reader on it and came up with

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

Checked coolant level, is right where it should be in tank and overflow bottle.

It's been cold (for Mississippi) last couple mornings and I noticed that my 15-20 min drive to work (35-40 MPH) truck only register say 160-180 on the temp gauge. On the way home around in the afternoon it reads up around the normal 200.


Quick net look says something about not reaching operating temp quick enough and normally means thermostat, which the way it reacts, doesn't seem that way.

Still runs great, no issues. on the truck. Heat works & everything that would lead to say a thermostat problem isn't registering with me.

You guys think thermostat, or is there something else that may effect it?

I was thinking about resetting the code to see if it throws it again, buy I wanted to check here first.

Thanks folks

[bwood]

It could be the thermostat sticking open (they usually stick closed though) not allowing the truck to warm up to operating temp. The coolant temp sensor (CTS) monitors the temp at the thermostat housing in the intake. The ECM looks at this sensor to change the truck from open to closed mode and many other adjustments. On the newer trucks in knows when the temp has been too low for too long and will register a code. The CTS could also be going bad and giving the ECM a false reading. Here is a scale to test the CTS with an Ohm meter:

Originally posted by ChevyTech:
Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C.

Just a FYI the guage gets it reading from a sensor mounted the drivers side cylinder head.

[STOCKISH]

Your code basically means coolant temp is too cold for closed loop operation. Very common on these trucks. Replace the thermostat and get rid of the Dexcool coolant if you havent already. Feel free to PM me if you have any other diagnostic questions--Josh

[truckdude239]

1972SuperCheyenne you go to wake forest? in in winston?

Shop key says to flush coolant and check the teromstat

[STOCKISH]

No I'm an ASE certified Master Tech @ a high line European repair shop in Apex NC.( I worked for the Chevy dealer for years) You would have to remove the thermostat to check it so it would be kind of crazy not to replace it while it was out. Make sure to buy a AC Delco part.

[67ChevyRedneck]

Bill, if you get rid of the dex cool, or have to top off the radiator, make sure you use an anti freeze that is compatable with dex cool, if you can't flush it out 100%. It will gel and make a nasty mess in your radiator/engine, etc. Also, I think dex cool is recommended to be flushed every 5 years/150K miles.

[Palf70Step]

Since its freezing (to us folks here) I'm going to tear into this weekend (I hope). I am pretty sure all the DEXcool is out of it, but "GUARANTEE" it will be when I change out the thermostat. I always flush my systems when I do anything that causes me to drain/partially drain the system (plus when I am suppose too).

Hearing you folks talk, I guess it could be stuck OPEN, heck it may not even have one in it, and it just didn't become an issue till we got REAL cold (for us) down here.

I just have to wait for it to warm up a hair, because the way things are in the shop, I can't get it inside to work on it. I am too old to be doing that cold fingers thing outside.

[67ChevyRedneck]

I looked at my haynes manual. It says for overcooling the thermostat is most likely stuck open. Follow the upper hose to the engine inake and that's where it is

[truckdude239]

was reading more about it code is set if the truck runs min of 2 mins then it has 15 mins to warm up to proper temps if it doesn't it sets the code

[Palf70Step]

I need to find where I put my book

This online service guide I got, ain't worth %^&$. I want to break down and get the full GM service manuals for it, so I guess I need to go surf'n.

[Josh]

It's the t-stat which is very common on the 4.3 trucks. Who do you use for the online manual? Best way to off the dex cool is to drain n fill with straight water and then drain and refill with water till the orangeish pink tint is gone, usually only takes once or twice to make it all go away.

[bwood]

There should also be a block drain on either side of the block near the motor mounts. One should have a plug in it and the other may have a knock sensor screwed in. These will drain the majority of the block and most of the heads. You will have a little in areas that just don't drain. Just use the Prestone 50/50 that mixes with both green and red coolants (about $8 @ Wally World).

Just a side note, I never use city water to fill any type of cooling system. It may still contain many menerals and chlorine (oxidizer) that could cause damage on its own. If you don't use the 50/50 stuff, use distilled water for the rest of the fill. The water here in Denison, TX is horrible. It runs about 300ppm hardness and can cause rusting in an iron block in as little as a year (my brother in law was using it in his car when the water pump started leaking). Its been flushed twice by a radaitor shop and is just now clearing up.

[Palf70Step]

Josh. I was using EAutoRepair.net. On my frienmds 94 truck and 2000 Buick we had gotten really decent and indepth GM manuals, but the ones for my 02 Silverado and my 96 Bonneville just lacked a whole lot. Seemed that everything was based around being you, a dealer mech with all the highend diagnsotic systems. So for me, the dude with just a code scanner, multimeter and backyard skills it was almost impossible to follow any of the flowcharts or work throughs. I bit down tonight and got the GM (paper) service manuals through Helms. They have never let me down, and my friend's book seems to be useful to a dude like me.

Oh yeah, the way you describe is how I normally flush mine out. I have a reverse flush setup somewhere, so if I ever find it again, I'll use it also.


bwood I have had the issue with city water before, but no issues from the stuff we have here.
I appreciate all the input. I just wanted to confirm what the scanner said and see what you folks have had to deal with. Now for it to get above the teens & twenties so I can do all this mess.

[Palf70Step]

Well, it warmed up here yesterday, so I tore into the old girl. I must say, it was a heck of a lot more work than I though changing out a thermostat would be. The buried that thing! All worked out though. She is flushed and all hooked up and running like she should.

I did notice that I don't think I ever want to change the wires or dist cap. I saw that wires went back there somewhere, but where they hid the distributor? Well that will be some other day.

Thank you guys for all the help and pointers.