DEF level sensor issue

steiny93 said:

After filling did it show full or still empty?

When towing (so using a lot of def) my wife's Denali will show 0 bars but still have more then 1/2 of a tank of def. When you fill it up it then shows a full tank of def. It has been to the dealer a couple times for this; all systems normal.

What is occurring is that the computer sees that it can't go more then 300 miles on the current amount of def so it sends the warning to the dash with a miles count down, when it does that it must think "well I warned them so I cannot show much def, I'm going to show you zero def" so the tank goes to zero even tho you have more then 1/2 a tank remaining.

When we tow for long distances (>1000 miles) this behavior occurs; doesn't happen on short trips (<250 miles).

Not sure if that is what is occurring for you; but this is supposedly a 'normal' behavior.

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We have this exact issue. Did you find a fix?

Our DEF usage skyrockets when towing. We get these results when towing, every time: warning at ~900 miles left (could be less or more, our first warning pops us), then an hour later (and 80 miles later), we are at ~200 miles left. Gauge goes from 3/4 sometime to close to empty. Then it will take 2.5 gallons. It's completely erratic and not reliable. This only happens when towing, but it happens pretty much the same way every time. THIS IS NEW: This week the vehicle shows us a DEF range each time we start it (haven't towed in over a week, DEF tank filled last week) and its slowly counting UP from 200 miles of range, to 525 this morning. We laugh each day that we get more range, even though it's being driven daily. Our hope is that when it reaches some threshold, it won't give us our range on starting.


Edit: The article above from GM stating that the "gauge" is an algorithm makes sense to me. Which would explain why the system gets erratic only while towing, because its trying to "do the math" on what's left based upon strain on the engine, plus things like altitude, outside temperature, etc. It's like the computer gets drunk while towing and "does its best" to "do the math". In a $90,000 vehicle. Are actual float sensors really that expensive these days?!?!

Secondly, and most importantly: Will the vehicle go into limp mode while there is still DEF in the tank, because of the erratic “gauge”? Has anyone has this happen?

BobHolthaus said:

We have this exact issue. Did you find a fix?

Our DEF usage skyrockets when towing. We get these results when towing, every time: warning at ~900 miles left (could be less or more, our first warning pops us), then an hour later (and 80 miles later), we are at ~200 miles left. Gauge goes from 3/4 sometime to close to empty. Then it will take 2.5 gallons. It's completely erratic and not reliable. This only happens when towing, but it happens pretty much the same way every time. THIS IS NEW: This week the vehicle shows us a DEF range each time we start it (haven't towed in over a week, DEF tank filled last week) and its slowly counting UP from 200 miles of range, to 525 this morning. We laugh each day that we get more range, even though it's being driven daily. Our hope is that when it reaches some threshold, it won't give us our range on starting.

Edit: The article above from GM stating that the "gauage" is an algorithm makes sense to me. Which would explain why the system gets erratic only while towing, because its trying to "do the math" on what's left based upon strain on the engine, plus things like altitude, outside temperature, etc. It's like the computer gets drunk while towing and "does its best" to "do the math". In a $90,000 vehicle. Are actual float sensors really that expensive these days?!?!

Secondly, and most importantly: Will the vehicle go into limp mode while there is still DEF in the tank, because of the erratic gauge? Has anyone has this happen?

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Regarding the fix question:
Per multiple dealers and GM; nothing to fix as it is operating as intended.
yep, seriously, that's what I've been told.

Regarding the; does it go into limp mode question:
Yep, it will, it takes a bit in that it will go further then it says it will; but yah, you'll get to limp mode joy.

Mine doesn't do the counting up behavior, that's a new one. I haven't used the GMC for towing in quite a while due to this unacceptable behavior, maybe that is from a software update. Great when empty tho.

steiny93 said:

Regarding the fix question:
Per multiple dealers and GM; nothing to fix as it is operating as intended.
yep, seriously, that's what I've been told.

Regarding the; does it go into limp mode question:
Yep, it will, it takes a bit in that it will go further then it says it will; but yah, you'll get to limp mode joy.

Mine doesn't do the counting up behavior, that's a new one. I haven't used the GMC for towing in quite a while due to this unacceptable behavior, maybe that is from a software update. Great when empty tho.

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Good to know on limp mode. I guess when towing (which for us is interstate driving typically across state lines), I’ll just know that when we get out first “range left” indicator on the DIC, that we need to top off the def tank at the very next fuel stop, and not wait. Thanks for the reply.

From what I understand the sensor is more like a "Sonar" and not a "Float" type level sensor. this it can be influenced easily high or low and is in exacting at best.
I dump a Wally world 2.5 in every 7500 miles roughly, and I've never been low and never been full....

Stbentoak said:

From what I understand the sensor is more like a "Sonar" and not a "Float" type level sensor. this it can be influenced easily high or low and is in exacting at best.
I dump a Wally world 2.5 in every 7500 miles roughly, and I've never been low and never been full....

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Read this. Makes sense to me. Sorry, this is what I referencing above; this was on a different thread. Added above too.

Update: we had the TSB done at the dealer. We are on our second leg of a 600 mile (each way) vacation pulling a 5100 pound boat and trailer. We got one “X miles to empty on def” notice, but the gauge was accurate. With 2 of 8 bars gone, we took 1.7 gallons (which is accurate). We only got one warning for miles and then we didn’t get another one. In our opinion, the TSB really helped the situation. See above for our erratic garage and mileage warnings of the past.

But we still filled at the next truck stop! Although we did drive 4 hours at 80 mph and only use 1/4 of a def tank, which is really good. 70 degrees at the start of the trip. It’s 88° now; anxious to see if def usage goes up in warmer temps.

BobHolthaus said:

Update: we had the TSB done at the dealer. We are on our second leg of a 600 mile (each way) vacation pulling a 5100 pound boat and trailer. We got one “X miles to empty on def” notice, but the gauge was accurate. With 2 of 8 bars gone, we took 1.7 gallons (which is accurate). We only got one warning for miles and then we didn’t get another one. In our opinion, the TSB really helped the situation. See above for our erratic garage and mileage warnings of the past.

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Which TSB was done....GM service bulletin 22-NA-150???

kerlyb said:

Which TSB was done....GM service bulletin 22-NA-150???

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Yes, that is the one.

Second update: I didn’t get another mileage warning until the tank was at 2 bars. Then it was like a red alert, ha. I filled the def tank at the next stop and it took 4.7 gallons. Another mileage warning would have been nice, although the gauge was reading correctly.

Hi everyone - for those that are getting faulty low readings, have the sensors actually limited your speed? My DEF tank was full to the brim last week and I only drove a couple hundred km’s and noticed my dash displayed my range to empty on DEF (1,950 km’s….) am now getting the low def reading on my GMC app. I haven’t got that error in the truck yet so I’m assuming the sensor is messed up.

I have a road trip to go on this weekend so don’t want to assume the sensor wont shut me down despite having a full tank and don’t want to get stranded. DEF tank isn’t leaking either - truck is parked in the garage so I’d see the fluid.

JayceeP said:

Hi everyone - for those that are getting faulty low readings, have the sensors actually limited your speed? My DEF tank was full to the brim last week and I only drove a couple hundred km’s and noticed my dash displayed my range to empty on DEF (1,950 km’s….) am now getting the low def reading on my GMC app. I haven’t got that error in the truck yet so I’m assuming the sensor is messed up.

I have a road trip to go on this weekend so don’t want to assume the sensor wont shut me down despite having a full tank and don’t want to get stranded. DEF tank isn’t leaking either - truck is parked in the garage so I’d see the fluid.

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There is a TSB for DEF level errors on GM trucks and SUVs. I think the TSB is 22-NA-150 (it's been updated to add new model years several times). It's a software update.
The level sensors in the DEF tank work the same way water & waste tank levels are in an RV (not very accurate). You can't have a float since the DEF is allowed to freeze in the tank.