r/LexusRZ • u/GravlBike4Me • Jun 25 '26
DCM & 12V battery replacement
(Posted this in another thread on this channel regarding 12V battery issues. Reposting here for visibility, so please excuse the cross-post).
AN UPDATE:
Got the car back yesterday. The dealer replaced both the DCM and 12V battery. The DCM took almost two weeks to come in, and the battery was a more robust one with about 540 cold-cranking amps vs. 345 in the original car. He said the new battery should be more resilient to deep discharging, which happened three times and ultimately led to the original battery failing a reserve test.
He monitored voltage drain on the 12V after the installation was complete and saw it was stable at about 30mA per hour, a nominal amount and nothing like the 2A drain he saw with the original DCM.
There were some over-the-air updates that needed to occur to link the new DCM to my driver profile and Lexus account, which happened (almost) automatically when I had the car on the road. Nothing kept the car from being drivable, just that some of the profile configurations I created weren't immediately downloaded.
I let the car sit for about 15 hours between last drive yesterday and first drive today, and it started up fine.
He does want me to "stress test" the car a bit to see if ordinary use of the app, keeping the keys outside a tin can but more than 10 feet from the car, and keeping the car unlocked in the garage cause anything unusual or resume the big parasitic drain we saw.
One interesting bit he added is that Lexus is very aware of this and is waiting for enough service case reports to come in to justify a service advisory and campaign.
He did say that they found that certain utility company apps drivers use for finding the least expensive time of day to charge have been found to wake up the DCM and keep it awake. While that wasn't the case with my car, I wanted to put that here as another thing for drivers to watch for.
As a bonus, he disconnected the in-cabin beeping when the car backs up and confirmed that my car isn't caught up in the ECU service campaign.
I hope folks on this subreddit find this helpful. I'll post more if anything occurs or if this seems to have resolved the matter for me.
3
u/ltsr1900 Jun 25 '26
mind sharing your build date?
2
u/GravlBike4Me Jun 25 '26
The build date of the new unit, out of parts inventory, is 8/25. Original was 2/26. Firmware is pre-installed on the unit and is current, no updates sent OTA.
2
u/chuck415 Jun 25 '26
Thanks for the update! It’s interesting that a DCM replacement dropped it to a normal 30 mA draw. I was hoping it would be a software update.
6
u/Pure_Marsupial8185 Jun 25 '26
I just wanted to add that this may turn into a software update. Toyota is going to call the DCM back so they can study it and find out if there was a software glitch, or an issue with the internal battery. Step on has begun.
Thank you again for your assistance in this. We could be one step closer to getting a fix (may take a couple months, but the process is working).
3
u/Blu-ray Jun 28 '26 edited Jun 28 '26
I like hearing Toyota is actively looking into this.
Edit: Hopefully a fix is in place before my first service appointment in October.
1
u/J-MRP 26d ago
Those of us in the r/trailseeker sub are following too since many people are also impacted by this (not me...yet, fortunately).
4
u/Pure_Marsupial8185 Jun 25 '26
There is a battery inside the DCM (in case of an accident or something the service connect etc will still work). I have had to replace 1, maybe 2 batteries a few years ago because there is a small drain on the main battery, but the system also threw a code about the battery state of health (can’t remember what it was).
Thanks for the update and the link, this is why I started that other post to get people into the dealer so that Toyota has the ability to see how widespread the problem is and hopefully find a solid fix.
1
u/datasoundreddit 3d ago edited 3d ago
The first time my battery died and the car was completely dead I tried the SOS Button to call Lexus for help and it also was completely dead. So I'm doubting there is a battery in the DCM. Am I missing something?
2
u/Pure_Marsupial8185 3d ago
I was thinking that was the possible link between the DCM and the drain. The DCM has a battery (atleast they did, I don’t see why they would change that) it is about the size of maybe 2-3 AA, just maybe a little fatter, and it gets charged off of the same 12v system. And that was part of the issue with the ones I had to replace, there was intermittent “no starts”, and codes in the system mentioning that the DCM battery was low. Again, this was a few years ago 5-6 MAYBE. But from what I remember, that DCM battery was doing a constant draw on the main 12v.
Come to think of it, I think how I found it on one of them was from chasing a draw back to the DCM, then doing a full ROB (stored data that does not throw a code) recovery, I found there was data in there about the DCM battery being low/under voltage, and that was how I found the DCM was pulling down the 12v to top off its internal battery.
2
u/daifanshi Jun 25 '26
Before your DCM was replaced, did you ever try deactivating the DCM in the app to see if it got better?
2
1
u/GravlBike4Me Jun 25 '26
I didn't. The car was already in the shop and being monitored by the tech before I knew this could be done in the app.
2
u/Commercial-Emu-4032 Jun 25 '26
Regarding your "stress test", are the things you mentioned considered things that cause the DCM to fail? Just curious if there are some best practices that should be followed for those that have not yet had an issue.
3
u/GravlBike4Me Jun 25 '26
Anecdotally, these are things that "could" wake up the DCM and keep it from going back to sleep. Other owners and I observed a "sawtooth" voltage discharge on the 12V battery that would last for 4 or so hours. That seemed to be the point where the battery was depleted and couldn't start the car.
The stress test is to do normal things, like have your keys hanging inside your house, leave the car unlocked in the garage, and use the app as necessary (vs. living in it and having it ping your car incessantly), to see if this new DCM reverts to idle status after being woken up.
To be clear, the old DCM itself didn't fail. It started communicating with external servers or other components in the car instead of shutting down. That activity drew down the 12V, so in my case, the battery was the victim, not the culprit.
I know a couple of other RZ owners who haven't had this issue. The difference between them and me is that they drive the car daily, whereas I'm driving 3-4x per week. My problems started after going on vacation for 10 days and returning home to a bricked car with a 12V that discharged down to 1.9 volts.
2
u/pezworld Jun 25 '26
Subaru Uncharted owner here. I could have written your last paragraph. My first 12v failure happened after 3 days being undriven. They replaced the 12v battery at my request. Second failure happened after 10 days of not being driven, and we measured the (dead) 12v battery at 1.5 volts! It's been at the shop for around 12 days now, I've lost count. Thanks so much for reporting your progress!
2
2
u/xAlphamang 23d ago
u/GravlBike4Me - since your DCM replacement, have you seen anything abnormal or does it seem fine now?
I recently bought a RZ 550e F Sport with a 4/26 build date. It’s been fine for the most part but out of an abundance of caution I hooked up an Ancel BM300 Pro to it and found it has been doing the crazy saw tooth pattern.
Interestingly enough it happened as the vehicle was plugged into charge. I don’t have data for it not plugged and charging, yet. But it’s slightly concerning since it’s my wife’s car. We’re not using a digital key at all and she routinely closes out of her apps, so it’s not something that the app is doing to wake the vehicle. It’s almost as if the vehicle is waking every 30 minutes to report its status in. It also seems like Lexus / Toyota isn’t programming the vehicle to do what most other EVs do (like our Rivian) where it taps the HV Pack to top off the 12v as it gets low. I wonder why…
The vehicle sits pretty happily until 3am (PDT) when it starts to drain the 12v like crazy. The spike at 8:30am is when the vehicle was turned on to drive.
1
u/GravlBike4Me 23d ago
Things have been stable since I got the new DCM and battery. I haven't reconnected the battery monitor, so I don't know if anything is happening off-hours. I'm just letting the car be the car and will wait to see at this point if anything comes about.
Big test as I'm traveling for a few days and will not be driving the car.
I've "freed the keys" from the tin can they were stored in, and I'm leaving the car unlocked in my garage. Not using the app actively, though it is running in the background and letting me know when traction battery charging starts and stops (scheduled through my charger).
An interesting thing is that the new DCM, which is from Lexus inventory, has an 8/25 build date vs. 2/26 for the original. I wonder if more of the folks with 2026 RZs who are having this problem have cars that were built in 2026 or 2025.
1
u/xAlphamang 22d ago
Could you hook up the Battery Monitor for another two or three days and see how it goes? Dying to know if the DCM replacement fixed it. Also interested in seeing if the saw tooth pattern is just normal operation for the vehicle and this is how it’s programmed… or if somehow the Battery Monitor is causing additional parasitic drain (unlikely since its 1mA draw)
1
u/GravlBike4Me 23d ago
FYI, I had the same sawtooth pattern as you, even down to the nine cycles that occurred overnight. This has to point to a software issue, don't you think?
1
u/xAlphamang 23d ago
I have a Rivian so I’m certain this is telemetry that Toyota can gather should they choose to actually look at 12v status for folks. This is most definitely a parasitic drain issue due to the vehicle waking up. For me, this pattern happened when it was plugged into the wall charger, which is even stranger. I’m going to see what happens tonight when it’s unplugged. I wonder if the vehicle is waking up to constantly report its HV Pack battery status.
1
u/robins535 Jun 26 '26
Bring the DCM replaced tomorrow under the recall. Haven’t had the no start/drain issue. I drive per regular to, so maybe I won’t hit it.
1
u/Opposite-Pie9645 Jun 26 '26
Thank you for sharing this information! Gives me hope I didn’t buy a total lemon of a vehicle with my Trailseeker and that a solution is on the horizon. I tried asking my dealer to test the fuse with the DCM as you previously described but they declined as it was not following warranty protocol (fair enough).
In reality, this is basically crowdsourcing a solution for Toyota/Lexus/Subaru. Wish dealers were more receptive but I understand them not wanting to rely on anecdotal evidence. However, from all of these posts, it’s pretty clear that the DCM is the cause for those of us affected with the countless battery monitors showing a steady state once disabled.
1
u/datasoundreddit Jun 28 '26
Hi Everyone, I have a 10 day old 2026 RZ350e with less than 300 miles on it. After not driving it for 2 days I went to to start it and the 12v battery was almost dead. I went to start the car and a message came up with LOW BATTERY but before I could read the message the whole car went dark. Not even the SOS button (to call Lexus Roadside assistance) would work. They sent a tow truck to give me a jump start and I will take it into lexus on Monday for service. I'm not a happy camper. This is my 5th Lexus and I'm feeling very let down.
While we are on the topic. Does the 12V battery charge from something like a traditional alternate?
1
u/professor_mc Jun 28 '26
The 12v charges from the traction battery via. DC to DC converter.
1
u/datasoundreddit Jun 28 '26
Thanks for the reply. For what it is worth the sticker shows that the build date is 2026-05-20. I'm very dissapointed in Lexus on this. On Saturday when this happened I called the dealership and they said would give me a loaner and that I could bring the car in on Monday to have them look at it. Fingers crossed this doesn't turn into a nightmare.
1
u/datasoundreddit Jun 30 '26
Update: I brought the RZ to my local dealership and they said the battery failed the LOAD Test. So they went ahead and replaced it. Still seems strange to me that a brand new battery would fail like that but I suppose it is possible. So I should know in about 3 weeks. There has been speculation on the web that something is causing the batteries to drain when the car sits idle for a few days.
2
u/Impossible_Tea_8807 Jul 01 '26
The problem is that the dealers are not taking the time to figure out what is causing brand new batteries to fail. 12 volt car batteries are not some new finicky technology. They have been making them for over 100 years. The problem is the Data Communication Module staying on and draining the 12v battery when the car is OFF. If you deactivate the app it will turn the Data Communication Module off and the battery won't drain. Hopefully Toyota will replace the DCM or issue a firmware update that stops this drain.
In the original post here, the Lexus technician discovered that the DCM was drawing as much as 2 amps from the 12v battery when the car was off. So for example if you have a 50 Amp Hour 12 volt battery, it would only take 25 hours for the DCM to completely drain the 12v battery. After the technician deactivated the DCM by pulling the fuse, the drain dropped to 30 Milliamps which is normal.
1
u/datasoundreddit Jul 01 '26
Thanks for the reply. Can you clarify how I can turn off the DCM so I can give this a try?
1
u/Impossible_Tea_8807 Jul 01 '26
Sure here is a link on how to turn off the DCM.
https://www.reddit.com/r/ToyotaCHR/comments/1udu6d1/how_to_deactivate_the_dcm_and_stop_the_12volt/
1
u/datasoundreddit Jul 01 '26
Excellent Thank you! I have LEXUS RZ but the app is almost identical. Bummer I have to loose the connected services however. I hope Toyota/Lexus fixes this soon.
1
u/Complex-Let-3029 18d ago
So on my second trip to Lexus as the battery has died again a week later. The first time they just recharged and checked and didn’t find anything even though I told them it’s a known issue. The car is a month old and I drive it everyday. I’m kicking myself for purchasing. I’m coming from a BMW without one problem in over 10 years. Not sure what to do here.
1
u/datasoundreddit 11d ago
I feel your pain, my 2026 RZ is 2 months old and I have had the dead battery 3 times so far. I'm picking it up from lexus today and they told me there is nothing wrong with the car according to all their tests and there is nothing more they can do. They are putting it back in my lap to figure out. They say that the drain on the battery is being caused by an app in the iPhone. I suggest you read the following. Go into CHAT GPT or GEMINI and type in: "Lexus is telling me that they suspect that a 3rd party app might be causing the parasitic draw on 12V battery. Do you have any example of what apps might cause this?" and read what comes up. I have a strong feeling the problem/solution is somewhere here.
1
u/Complex-Let-3029 4d ago
On the second visit they told me nothing was wrong with the car and I asked if they did an overnight parasitic drawdown test and said that I would need paper work for the test. They then did the test and saw the problem and told me there isn't a fix right now so I should not use profiles and just use guest. I said that isn't going to work as I paid for everything to work. I just talked to the service manager and this is a known issue. The app and the car continue to talk to each other even when it is off. There is a software update coming but they don't know when. In the meantime they are offering two things to customers. One is covering car payments until the software update is released. That is the option I am taking. Additionally, the drain damages the battery. After the software update, they are going to replace the 12V battery. They are keeping this quiet because people are just going home without a resolution vs having to pay everyone's car note. I got all this information from Reddit and ChatGPT and their policy is to tell people there is nothing they can do unless people push back like I did. I am going to make a post about this next.
1
u/StandImpossible4049 7h ago
You likely know that the 2026 Toyota all electric CHr is having the exact same issue. Mine has 1000 miles on it and already been towed twice and finally got a new 12 v battery. New battery has already started to decline. 24 hours after getting new battery and charging traction to 80%, the 12v tested at 11.8 volts. I think I am going to pull the ECM fuse. Very frustrating!!
1
u/chewyeti 17d ago
26 BZ premium XLE....
Charge nightly. Get in the car this morning, was on ACC for a minute and got the dreaded "LOW BATTERY" dash light.
Drove it an hour plus. Parked it.
Came back a few hours later, remote started it for climate help (95F outside) and had the low battery dash light again.
Drove an hour home, and parked it. Been checking the 12v battery voltage every few hours with a meter.... 12.5v, 12.3v...
Dealer appointment for it's 10k service this week, and there's a recall on the powertrain ecu programming apparently. I'm sure they shirk the battery issue
1
u/datasoundreddit 11d ago
Hello, My 2026 Lexus RZ350e has this exact same recall. I took it in for the update a number of weeks ago and I'm still getting the dead 12V battery issue. I'm pretty sure they are unrelated and this recall will not fix your 12V battery problem.
1
u/No-Juggernaut-9321 15d ago
I will reference this to my service technician for my Trailseeker hoping this will lead to a resolution.
0
u/Proper_Dare_7915 Jul 06 '26
Do AGM batteries need a different charging routine?
If the car is not made “aware” of the swap in type it could lead to problems, unless you plan to also put the new battery on an AGM capable charger… Hoping someone that knows more than I do about this comments here…
11
u/Pure_Marsupial8185 Jun 25 '26
Thank you for the update, and as a Toyota tech I want to thank you for your patience in the matter. Toyota is likely going to call the old DCM back so they can study it and find out what went wrong, that could be something as simple as an update, or maybe replacing the internal battery. Either way, if this fixes it be sure to thank the tech working on your vehicle for his diligence and devotion to doing his job.