I am new to owning a Duramax. I just bought a 2019 with 23,000 miles and I have the low coolant warning everything I start the engine. I have filled the lower reservoir 3 times and it slowly looses coolant. I found what looks like evidence of a leak (oil or coolant) right under the starter and running down the transmission to the lowest point. I also see evidence of oil or coolant on a flat black metal cover on the passenger side of the tranny. I'm not sure what the cover is there for but it has about 6 bolts holding it on. Here are the pics. The first is the stain under the starter and the second is a pic of the cover plate on the side of the tranny.

 

Bottom chamber is not supposed to have very much in it. Only top should be almost full. The bottom acts like a recovery if coolant gets too hot. Bottom cap on tank is not supposed to be removed for service- only the top. Sounds like your level sensor is bad. Next time it does it unplug the sensor and see if the warning goes away. You can put a paperclip in the connector to short the pins and light should come on.
The stains look like oil and the plate is probably the cover for the pto mounting window.

 

Sorry. The light should go out with the pins shorted

 

Ok thanks I will try that. I filled the bottom part of the container up to the weep hole that is behind the pressure cap down at the bottom fender side of the tank.

I just located the sensor wiring/plug this afternoon so I will unplug and jump it the next time I see the warning message- that's a good idea so thanks for the tip.

On another note, why in heck do I have an oil leak on an engine that has 23,000 miles on it?

 

Why not take it to the dealer and have both problems fixed under warranty?

 

I took my 2019 Duramax to the dealer today for a "coolant level low" fault, they said that the coolant level was not low and the sensor was fine... they suggested I replace the coolant tank... which to me is stupid, the thing holds pressure... I think they were just after 400+ dollars, told them no, I'd fix it myself. They also told me it was not covered under any warranty... less than 42000 miles on it.

 

…………. My Low Coolant light came on and noticed a leak from back of engine. Dealership had to remove transmission & oil pan to get to the rear of the engine block & it turned out to be an “O” ring leaking ; lots of minor parts replaced except for the oil pan as it’s a 1 time throw away part… Need to take it back because they “forgot “ to put all the parts back!….

 

wow, that's messed up... LOL

 

Just had mine come on again. Was breaking hard and must’ve sloshed enough to set it off. Added about 2 cups to bring it up to the top of the arrows. Engine was cold when added. At about 31K now. Looking at this thread, I last added some around 11K back in 2020. Guess I can live with that, but makes me think it must be going somewhere. Not worth a dealer visit though.


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Well, guess it’s officially a problem. Just drove a couple hundred miles towing and had to cancel the low coolant warning about 20 times. Look s under the hood midway and coolant park was up as high possible except a giant bubble at the top.

We have more traveling ahead the next couple days. Correct me if I’m wrong, but this will not create a limp situation. It’ll just be a nuisance as far as I can tell.


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Well, guess it’s officially a problem. Just drove a couple hundred miles towing and had to cancel the low coolant warning about 20 times. Look s under the hood midway and coolant park was up as high possible except a giant bubble at the top.

We have more traveling ahead the next couple days. Correct me if I’m wrong, but this will not create a limp situation. It’ll just be a nuisance as far as I can tell.


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Correct. If you aren’t low on coolant when the engine is warm then it’s probably just the sensor in the tank. It won’t cause limp mode.

 

Thank god it doesn't cause limp mode. Mine has been bitching at me for about a month. Got the tank already, just need the time to replace it.

 

Guess it gives me something do to on a long drive. Thanks GM.


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Guess it gives me something do to on a long drive. Thanks GM.


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Just curious, but can you see if your bottom portion of the coolant reservoir is full too? There's a few videos on YouTube that insist if you get that bottom half filled back up the lamp will stop illuminating. To fill that bottom half you need a small diameter plastic tube and some patience.

 

Not sure. At a campground now, and was going to check it post coffee now that it’s sat overnight. It’s on a bit of a slope, but I should be able to get an idea at least. Thanks for the tip.


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Should the bottom be filled when cold? Mine does look empty.


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Do not fill the bottom chamber. That is the overflow side, just like the old surge tanks were. If it needs to move excess fluid it will vent it to the lower chamber and take it back when it cools. The sensor only reads the upper chamber. It is a common problem for it to fail and the entire tank has to be replaced. About $120 at the dealers.

 

That's what I thought too, @sambor. But I noticed my lower tank level was empty as well, and the top was full. So I would let the truck cool down a little but still have pressure in the lines, then crack the cap a little to try and force some fluid from the top down to the bottom. Every time it would only go out the tube to spill on the ground instead.

I'm guessing I really do not understand how the tank works in the L5P, because I would think that process would force coolant from the top tank to the bottom for the overflow.

 

If the upper tank is overfilled it pushes coolant to the lower. If the lower fills up it pisses out on the ground.

 

That would make sense.

 

I have a 2019 2500 HD for the Duramax Diesel with 16 just turned 16,000 miles doing the same thing The jug is completely full of top and the light keeps or the gauge keeps telling me I'm low on fluid I haven't got it to the dealer yet it's still under warranty I believe

 

Bad level sensor

 

Get the tank with sensor replaced. It's the only solution.
When I replaced mine, level sensor was corroded.

 

My 2019 GMC Denali 2500HD has 29,000 miles and starting having issues yesterday. The low coolant light came on and It did smell like something was burning. Time to take it in.

 

Burning smell isn't good. I hope it was only a regen in progress though. I still need to get my 2018 in for the faulty coolant sensor. <sigh>

 

I feel the pain guys, my message has been on for just about 2 months. I already have the new tank & coolant sitting in the garage, I just need a weather and time window to get the job done.
Just a double check here, you guys are making sure the coolant level is above the horizontal baffle line when the truck is dead cold, right?

 

Just a double check here, you guys are making sure the coolant level is above the horizontal baffle line when the truck is dead cold, right?

My low coolant light just came on yesterday. It was actually a little low. I used This link to reference when topping it off. I topped it off and so far no light again.

 

Yup, that's the correct link. Figure 7 shows a low level.

The important take away here is don't fill it to the bottom of the neck cold.
It's tough to see in the pictures, but figure 8 shows the horizontal baffle that runs across the tank. That's what the level needs to stay above at a minimum.

 

Just dropped mine off a few days ago at the dealer since I’m working this week. Upper tank is always full, but lower tank is always empty. Hopefully they’ll throw a new tank in there. We’ll see though.


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Upper is supposed to be full and lower almost empty- its the overflow side.

 

Ok. Well, probably new tank anyway. I should’ve ordered a tank and did it myself to avoid my truck sitting there a week, but with 4K left on bumper to bumper, I figured wth. Let them do it. Just for sport I asked about my lack of increasing def warnings. I know they’ll never figure that one out.


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