Low battery / FOB-electronics not working - How to open tailgate manually for unload/jumper cable access?

[tagexpcom]

Hey All

I have a 2021 Yukon Denali w/6.2L. I went a little too long (week) and the battery went down so low (or in some kind of protect? mode) the FOB would not click/open the doors in the normal matter. Was able to open the front door, tried to start a couple of times but just a chug, would not turn over. Then thought, 'battery cables' ....

But they're in the way back compartment and it was loaded with stuff blocking access to jumper cables. Fortunately this was at home and I had another set of cables.

Can the rear hatch be opened manually with low/dead battery?

I'm posting because this was an eye opener for me. Rear doors would not unlock/open with FOB - had to reach over/around the front seat to toggle the rear door-open handle manually. This worked but could not have unloaded the back / gotten to the jumper cables this way as 'stuff' was too big/awkward to come out that way and I can't climb over. Jumper cables are no good if they can't be accessed!

 

[Joseph Garcia]

Is there a small circle on the plastic rear door interior covering in the middle (left to right) and a few inches above the bottom? In earlier models, this circle existed, and if you drilled out this circle with a 1/4" drill bit, you could insert a long thin screwdriver and manually unlatch the rear door.

 

[Fless]

All I can say is to keep the jumper cables handy.

 

[jfoj]

If your battery is original, it needs to be replaced, I can go weeks without driving my Yukon without a problem so far, but mine is a August 2024 build date, so the battery is probably less than 2 years from date of production.

Better yet, just get one of these and keep it handy, under a seat or wrap it up and see if you can stash it under the hood somewhere safe.

Amazon.com

 

[Doubeleive]

ya if it is going to be sitting a lot and you can run a extension cord to it I would just wire up a little maintainer and then keep the cord wrapped up with a small bungee cable under the hood, then all you have to do is plug the cord in when it is going to sit, there cheap like $30 all day long, easy solution....

 

[tagexpcom]

Is there a small circle on the plastic rear door interior covering in the middle (left to right) and a few inches above the bottom? In earlier models, this circle existed, and if you drilled out this circle with a 1/4" drill bit, you could insert a long thin screwdriver and manually unlatch the rear door.

I see it.... and appreciate the info!

But in this case it was blocked with tons of camping gear, electric bike, etc.

I guess I'm just shocked that there's no mechanical lever to open the hatch - on the outside like my SRX or the Yukon doors would be nice. Never occurred to me that this would not be an option.

 

[tagexpcom]

All I can say is to keep the jumper cables handy.

Yep, going to find a spot more up front for sure!

 

[tagexpcom]

Battery wise - I replaced it 2 yrs ago when I bought the rig. Fooled around with things quite a bit and I do have a float charger but it's been going 7 days lately.

I have a battery monitor on it and showed a slow decline to 11.4v after 6 days (which is OK) but then on day 7 it dropped from 11.4v at 6pm to ~10.4v by 8am the next morning. Not enough to start the car. Why it did whole volt drop in 12hrs I don't know.

Even at 10.0v, why wouldn't the FOB work to unlock doors - what the heck?

But then it get's worse, after trying to start a couple of times I jumped to an external battery (at 13.1v) but it didn't work and... slapped on a charger... and somewhere in all this the electronics went CRAZY!

Alarm went off, lights toggling on/off, relays clicking - had to disconnect the negative post to kill the electronics. Even after charging the battery for 20mins, reset/re-connect negative, and getting things started - I had an engine check light (exhaust system code) that I had to clear - good to carry an ODBC tool.

Not sure modern electronics are all that user friendly for 'dead batteries' like the old days.

 

[Joseph Garcia]

I see it.... and appreciate the info!
View attachment 472095
But in this case it was blocked with tons of camping gear, electric bike, etc.

I guess I'm just shocked that there's no mechanical lever to open the hatch - on the outside like my SRX or the Yukon doors would be nice. Never occurred to me that this would not be an option.

Ahhhh.... GM finally put an actual removable plug there. No more drilling needed.

 

[Joseph Garcia]

They abso

Battery wise - I replaced it 2 yrs ago when I bought the rig. Fooled around with things quite a bit and I do have a float charger but it's been going 7 days lately.

I have a battery monitor on it and showed a slow decline to 11.4v after 6 days (which is OK) but then on day 7 it dropped from 11.4v at 6pm to ~10.4v by 8am the next morning. Not enough to start the car. Why it did whole volt drop in 12hrs I don't know.
View attachment 472098


Even at 10.0v, why wouldn't the FOB work to unlock doors - what the heck?

But then it get's worse, after trying to start a couple of times I jumped to an external battery (at 13.1v) but it didn't work and... slapped on a charger... and somewhere in all this the electronics went CRAZY!

Alarm went off, lights toggling on/off, relays clicking - had to disconnect the negative post to kill the electronics. Even after charging the battery for 20mins, reset/re-connect negative, and getting things started - I had an engine check light (exhaust system code) that I had to clear - good to carry an ODBC tool.

Not sure modern electronics are all that user friendly for 'dead batteries' like the old days.

They are absolutely are not friendly on these rigs. A weak battery triggers up the trouble lights like a Christmas tree, and all sorts of wonky things occur. I keep a spare battery in the second battery tray with high amperage quick disconnects, just in case that I get a battery failure somewhere a long ways from home or a repair shop.

 

[Doubeleive]

Battery wise - I replaced it 2 yrs ago when I bought the rig. Fooled around with things quite a bit and I do have a float charger but it's been going 7 days lately.

I have a battery monitor on it and showed a slow decline to 11.4v after 6 days (which is OK) but then on day 7 it dropped from 11.4v at 6pm to ~10.4v by 8am the next morning. Not enough to start the car. Why it did whole volt drop in 12hrs I don't know.
View attachment 472098


Even at 10.0v, why wouldn't the FOB work to unlock doors - what the heck?

But then it get's worse, after trying to start a couple of times I jumped to an external battery (at 13.1v) but it didn't work and... slapped on a charger... and somewhere in all this the electronics went CRAZY!

Alarm went off, lights toggling on/off, relays clicking - had to disconnect the negative post to kill the electronics. Even after charging the battery for 20mins, reset/re-connect negative, and getting things started - I had an engine check light (exhaust system code) that I had to clear - good to carry an ODBC tool.

Not sure modern electronics are all that user friendly for 'dead batteries' like the old days.

GM considers 10.5v the bare minimum voltage required
batteries are affected by weather, so when the seasons change a lot of batteries will take a dump the expansion and contraction from heat/cold might be minute but that's all it takes. when you hear someone say my battery lasted 10 years, well that was probably garage kept in a moderate climate.
and low voltage causes a bad handshake between the various modules in the vehicle, hence all the lights and errors

 

[jfoj]

Battery wise - I replaced it 2 yrs ago when I bought the rig. Fooled around with things quite a bit and I do have a float charger but it's been going 7 days lately.

I have a battery monitor on it and showed a slow decline to 11.4v after 6 days (which is OK) but then on day 7 it dropped from 11.4v at 6pm to ~10.4v by 8am the next morning. Not enough to start the car. Why it did whole volt drop in 12hrs I don't know.
View attachment 472098


Even at 10.0v, why wouldn't the FOB work to unlock doors - what the heck?

But then it get's worse, after trying to start a couple of times I jumped to an external battery (at 13.1v) but it didn't work and... slapped on a charger... and somewhere in all this the electronics went CRAZY!

Alarm went off, lights toggling on/off, relays clicking - had to disconnect the negative post to kill the electronics. Even after charging the battery for 20mins, reset/re-connect negative, and getting things started - I had an engine check light (exhaust system code) that I had to clear - good to carry an ODBC tool.

Not sure modern electronics are all that user friendly for 'dead batteries' like the old days.

What happens is all the computer modules need to power up and this puts a fair draw on the battery. Additionally all the modules need to communicate with each other and with low Voltage the modules cannot properly communicate. Then if the module Voltage drops too low, you can get the firmware/software in an "Invalid Logic State" so then the module will not process instructions correctly. This is why disconnecting the battery ad sometimes even touching the battery cables together for 30 seconds is needed, it discharges all the capacitors in the modules and puts the modules back in a valid Logic state assuming you show up with a good battery.

Your battery is either trash and/or you have a high parasitic draw on your truck. I hear you say the battery is only 2 years old, but a new battery can be bad. It needs to be properly load tested and have the internal resistance checked. A fully charged battery depending on chemistry is between 12.6-12.8 Volts.

Your graph shows the battery Voltage started at around 11.5 Volts which is a fully depleted battery with next to no capacity, 11.9 Volts is consider 0% capacity on a typical automotive battery.. Either your vehicle has a charging problem, you do not drive the vehicle much or your battery monitor is not reading a proper Voltage and you need a correction factor added to what is being measured. In case you were unaware a 12 Volt automotive battery that is at 12.2 Volts the battery is only about 50% capacity, 12.0 Volts the battery is only around 25% capacity.

Also note that since around 2009 these vehicle are designed to turn the alternator off at times and run fully on the battery. So be careful checking and testing the charging system, you typically need to turn the headlights on to make sure the alternator is fully enabled.

The other thing to understand the cranking Voltage should never drop below 10.6 Volts, a strong battery and properly operating starting system the cranking Voltage should typically stay at or above 11 Volts depending on the battery state of charge and ambient temperatures.

What tool/software/interface are you using to monitor the battery Voltage?

The reason the battery Voltage dropped to 10 Volts at the end of your graph is the battery has NO capacity and it avalanched, totally acceptable and expect, especially with modules having a parasitic draw. Understand that at least the keyfob receiver and the OnStar module are constantly "Waking Up" looking for commands to unlock the vehicle or remotely start the engine and possibly providing vehicle location and status. 10 Volts is below the threshold for most of the modules to function and the battery does not have enough capacity to unlock the doors anyway.

If the vehicle sits for over 2 weeks at a time, you need a Battery Tender. But you also need to fully charge your battery and have it tested to determine if it is still good. I replace batteries when the capacity indicates it is only 75-80% of its rated capacity. Not wort the headache of a no start either in the driveway or while on the road. Sure you may have free Roadside Assistance still, but usually you have to wait HOURS before someone shows up and often you may get a jump start but the probably battery is not replaced.

 

[tagexpcom]

What happens is all the computer modules need to power up and this puts a fair draw on the battery. Additionally all the modules need to communicate with each other and with low Voltage the modules cannot properly communicate. Then if the module Voltage drops too low, you can get the firmware/software in an "Invalid Logic State" so then the module will not process instructions correctly. This is why disconnecting the battery ad sometimes even touching the battery cables together for 30 seconds is needed, it discharges all the capacitors in the modules and puts the modules back in a valid Logic state assuming you show up with a good battery.

Your battery is either trash and/or you have a high parasitic draw on your truck. I hear you say the battery is only 2 years old, but a new battery can be bad. It needs to be properly load tested and have the internal resistance checked. A fully charged battery depending on chemistry is between 12.6-12.8 Volts.

Your graph shows the battery Voltage started at around 11.5 Volts which is a fully depleted battery with next to no capacity, 11.9 Volts is consider 0% capacity on a typical automotive battery.. Either your vehicle has a charging problem, you do not drive the vehicle much or your battery monitor is not reading a proper Voltage and you need a correction factor added to what is being measured. In case you were unaware a 12 Volt automotive battery that is at 12.2 Volts the battery is only about 50% capacity, 12.0 Volts the battery is only around 25% capacity.

After a drive / fully charged the battery shows 12.5v per my measurements. It gradually dropped from 12.5v to 11.4v over 6 days.

What tool/software/interface are you using to monitor the battery Voltage?

Using an ANCEL BM300 - https://www.amazon.com/dp/B07WCW49YM - as the battery monitor / where the info above came from.

Drove the rig for 2 x 30 miles yesterday and shows 12.5v immediately after parking.

Just looked at ANCEL + Voltmeter (for reference) and ANCEL seems reasonably accurate.... 12.21v vs 12.25 this morning at 11:30am.

The reason the battery Voltage dropped to 10 Volts at the end of your graph is the battery has NO capacity and it avalanched, totally acceptable and expect, especially with modules having a parasitic draw. Understand that at least the keyfob receiver and the OnStar module are constantly "Waking Up" looking for commands to unlock the vehicle or remotely start the engine and possibly providing vehicle location and status. 10 Volts is below the threshold for most of the modules to function and the battery does not have enough capacity to unlock the doors anyway.

If the vehicle sits for over 2 weeks at a time, you need a Battery Tender. But you also need to fully charge your battery and have it tested to determine if it is still good. I replace batteries when the capacity indicates it is only 75-80% of its rated capacity. Not wort the headache of a no start either in the driveway or while on the road. Sure you may have free Roadside Assistance still, but usually you have to wait HOURS before someone shows up and often you may get a jump start but the probably battery is not replaced.

Really appreciate you taking time to verbalize all this. It's in line with my experiences!

I also have no quarrel with voltage/% you talk about - agree. However.....

The ANCEL shows it charging between 14.4v and 14.6v while driving highway speeds. Just did 2 x 30minute drives yesterday showing the 14.5v charge - so no reason the battery shouldn't be fully charged on arrival home but yet it drops to 12.5v as the highest right after turning off the engine.

This lower voltage along with parasitic drain means I don't have 7 days reliably. Sometimes I do but weeks I don't - meaning 7 days is right on the edge.

This isn't 'crazy bad' but it's annoying compared to my previous cars from older vintages.

This newer 2yr old battery is behaving the same as the original battery that came with the rig when I bought it 2 years ago - it's why I bought a new battery as I suspected the old one was worn down. However, the current ('new one') behaved the same (voltage wise) as the old one right from the beginning - so I'm struggling to think bad battery.

--------------
Note: This 2021 Yukon Denali doesn't have a place for a 2nd battery, so I can't (easily) go that route (as mentioned above for backup) but I would if I could.

 

[jfoj]

You can't rely on the vehicle to fully charge the battery. Even though you're seeing 14.4 to 14.6 Volts that means the charging system's doing what it should but understand there's a high load on the electrical system while driving the vehicle. If it was mine I would go to the parts store and get the battery tested. But be careful because I always love how the battery test comes back over 100% capacity at times which tells me either the parts store people didn't configure the tester for the proper battery, cold cranking amp or amp hour setting or the software and the tester is not good. But I would personally condemn the battery if it didn't meet at least 80% capacity. Additionally, you need to get an external battery charger and charge the battery at least 24 hours to make sure it's really fully charged. Be careful of these small, cheap Chinese battery chargers that are only slightly bigger than a laptop charger. In my opinion they don't really do a very good job.

You cannot assume the battery is either good or bad just by looking at Voltage or battery age. It needs to be properly tested to determine whether it's has the ability to provide the rated capacity and doesn't have too much internal resistance.

Then if the battery tests good and we believe the charging system is working correctly then you need to look for a parasitic draw. One thing you may want to do is take the key fobs and put them in a metal container far away from the vehicle because sometimes when the key fob's too close to the vehicle say in side the house just inside the garage. It may cause the vehicle to keep waking up and put in excessive draw on the battery when it shouldn't be.

 

[tagexpcom]

Bottom line for me: Have moved the jumper cables from the back to under the front seat for access! and put a wrench in glove box so I can unscrew the battery negative to be able reset the electronics.

Appreciate the discussion/responses - thank you.

 

[jfoj]

Most good batteries that come off a charger or have been in the vehicle running have what is called a "Surface Charge" right after the battery is removed from the charger or the vehicle is shut down. Even with many modules operating in the vehicle when the engine is turned off, I would expect that the battery Voltage would not drop immediately to 12.5 Volt, I would expect the Voltage to be in the 12.8-13.2 Volt range. Even if there were modules powered on when the vehicle was shut down, once the modules go to sleep, typically after 20 minutes, the battery Voltage should recover and come back up to 12.6 Volts.

Not sure your apprehension to actually get the battery fully charged and properly tested? Worrying about jumper cables is not a way to live your life. You still need a donor vehicle if the vehicle will not start. I keep either a super capacitor or jumper battery in or near me at all times. My 2024 Yukon can sit for 3 weeks at a time and start no problem. Again, unclear what your local temperatures are or what the typical vehicle usage is.

I even put a "Maintenance" charge on my vehicles typically a few times a year as well as test my batteries for capacity and tend to replace them when their rated capacity is between 70-80% depending on the vehicle and the required reliability I need. While I have jumper boxes, super capacitors and a number of battery chargers, I still prefer not to need to even need to use them, they are an insurance policy for me and for me to help others out.

 

[NELLY1947]

Well, is it just these Yukons have a garbage electrical system . I have a 2023 that has been in the shop for atleast 7-8 times for the battery going dead. 2weeks ago was the best. Truck in the garage. Heated. After 7-8 days the truck was at 10.6 and would not start. Went in and they could not find anything. Brought it home and after 6 days dead . I tested it and it wss 4.5 volts. The booster battery could not do anything. So charged it and got it started. Finally for the first time a check engine light. Guess what...... at least 20+ codes as low battery with no reason. This is nuts. I have my wife's car a 2019 jeep cherokee that can sit all winter, 2-3 months and start no problem. Too bad we don't have a lemon law in Canada.

 

[LegalBrief]

After a drive / fully charged the battery shows 12.5v per my measurements. It gradually dropped from 12.5v to 11.4v over 6 days.


Using an ANCEL BM300 - https://www.amazon.com/dp/B07WCW49YM - as the battery monitor / where the info above came from.
View attachment 472297

Drove the rig for 2 x 30 miles yesterday and shows 12.5v immediately after parking.
View attachment 472299

Just looked at ANCEL + Voltmeter (for reference) and ANCEL seems reasonably accurate.... 12.21v vs 12.25 this morning at 11:30am.
View attachment 472298



Really appreciate you taking time to verbalize all this. It's in line with my experiences!

I also have no quarrel with voltage/% you talk about - agree. However.....

The ANCEL shows it charging between 14.4v and 14.6v while driving highway speeds. Just did 2 x 30minute drives yesterday showing the 14.5v charge - so no reason the battery shouldn't be fully charged on arrival home but yet it drops to 12.5v as the highest right after turning off the engine.

This lower voltage along with parasitic drain means I don't have 7 days reliably. Sometimes I do but weeks I don't - meaning 7 days is right on the edge.

This isn't 'crazy bad' but it's annoying compared to my previous cars from older vintages.

This newer 2yr old battery is behaving the same as the original battery that came with the rig when I bought it 2 years ago - it's why I bought a new battery as I suspected the old one was worn down. However, the current ('new one') behaved the same (voltage wise) as the old one right from the beginning - so I'm struggling to think bad battery.

--------------
Note: This 2021 Yukon Denali doesn't have a place for a 2nd battery, so I can't (easily) go that route (as mentioned above for backup) but I would if I could.

Really.. you have running boards, approach lighting and all that rich electronics.. they are battery pirates (vampires)

 

[jfoj]

@NELLY1947

Your problem could be the key fob proximity and/or and OnStar issue. If dealer cannot find a problem it may be due to them leaving the vehicle outside and the key fob more than 20-25 feet from the vehicle. The vehicle keyfob receiver and OnStar unit keep waking up looking for commands. I do not know how the location feature works but assume the GPS location is also checked often even when sitting.

So if you have a weak cell phone signal in the garage, the GPS cannot obtain lock or your key is too close to the vehicle, this could be part of the problem. I have also seen cases where OnStar notifies you the battery is low and tells you to start and run the engine. Have you ever received one of these messages?

Other issues could be with radio or vehicle updates hanging is a cue and not installed, 3rd row seat switch loose or disconnected, leaving the keyfob in the vehicle or a problem where a module update is required. I would hope the dealer would have already addressed this.

If the battery drain is more than 200 mA the vehicle is not going to sleep. If you lock your vehicle while in the garage, you might try leaving it unlocked as a test.

If you garage your and do not drive for a week or more, a battery tender would be a wise idea as well.

 

[WalleyeMikeIII]

Perhaps you have this issue -- may be worth asking your dealer.

https://static.nhtsa.gov/odi/tsbs/2025/MC-11023353-0001.pdf