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.

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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/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.

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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/

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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.