CMU Failure After Battery Replacement – Need Help Confirming DTC U3000:42

[dr_terapeut]

Hi all,

I’m hoping someone with Mazda service experience or diagnostic knowledge can help confirm or provide more context for a specific issue and DTC I’m dealing with.

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Vehicle Details: • Make/Model: Mazda CX-3 • Year: 2018 • Engine: 1.5 Diesel (Europe) • VIN: JMZDK6W601321772 • Mileage: ~90,000 km

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Issue: • I had the car battery replaced in May at an authorized Mazda dealership. • A few days later, the infotainment screen (CMU) stopped working completely (black screen, no response). • I didn’t have time to get it checked until recently because from that dealership said to me that there is no any connection to the battery, and that new display is expensive.

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Diagnosis (done July 30, 2025):

I took the car to another official Mazda service center. They charged for diagnostics and provided the following DTC:

• Module: CMU (Connectivity Master Unit)
• DTC: U3000:42-08
• Description: Control Module – General Memory Failure
• Status: 08 (Not Current DTC)
• Diagnostic Protocol: 14229
I’ve attached a photo of the actual diagnostic screen

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What I’m trying to understand: 1. Is U3000:42 a known memory corruption fault related to low voltage or improper shutdown? 2. Could this realistically happen due to battery replacement without proper procedure (e.g., no memory saver, surge protection, or post-replacement programming)? 3. Is it true that this DTC appears in CMU, not in blind spot sensors (as one service advisor tried to claim)? 4. Would it make sense that Start-Stop stopped working immediately after battery replacement, and the CMU died shortly after?

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What I suspect:

I believe the CMU failed due to a voltage spike or improper shutdown when the battery was replaced. This is backed by: • The DTC description itself • The timing of the failure • The fact that Start-Stop didn’t work after the battery swap • The CMU was working perfectly before

The service center is denying any link between the battery and CMU failure and claims U3000:42 is “related to blind spot monitoring” (which seems false, based on the screenshot and research I’ve done).

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Any insights or confirmations from Mazda techs or experienced mechanics would be hugely appreciated.

Would love to know if you’ve seen similar cases — or if you have access to Mazda technical documentation confirming what U3000:42 actually means.

Thanks in advance!

 

[TheMAN]

if you didn't bring your car back soon after the issue arose, the dealer can say whatever they want and deny this issue is related to the battery... anything could've happened over 3 months, so there's no reason to believe you without evidence

whatever your reasons were, you should've made time or come up with some way and brought the car back right away

but this is what the shop manual says:

 

[dr_terapeut]

if you didn't bring your car back soon after the issue arose, the dealer can say whatever they want and deny this issue is related to the battery... anything could've happened over 3 months, so there's no reason to believe you without evidence

whatever your reasons were, you should've made time or come up with some way and brought the car back right away

but this is what the shop manual says:
View attachment 343087

Thanks for the answer, I’ve called them few days after but they said it has nothing to do wit battery that displayed probably died and it’s expensive so I decided drive it without display for a while

Is this official mazda service book? Cause then it’s point blank that guy from dealership is lying, with blind spot detectors etc

 

[SparkYellow]

The issue you're dealing with is an advanced one. I just want to share with you my experience in battery replacement. It is very straight forward, engine off, disconnect ground, disconnect positive, battery out. Clean the terminals. New battery in, connect positive, connect ground. My newest car is this 2017 CX-3.

I like to DIY because it saves time. I am not familiar with any post-replacement programming and have not done so for any of the 3 replacements that I did. However, a mechanic told me that there is a sequence to reprogram the battery, and the instructions he had included something about i-stop so I believed it did not apply to mine.

Each time after the battery was replaced, there was no irregularity with any aspect of the electrical system. Something else is likely to have been impacted (or tempered with) in the process for your CMU to cease functioning. Could have been inadvertent.

Can you check the fuse box to see if all the fuses/relays are all present and tucked in? There are also two modules between the back of the battery and the firewall, they are the brain to the car. Can you check if they are both firmly seated and locked in?

These may or may not be related to your problem but can be checked before returning to the dealership.

 

[SparkYellow]

if you didn't bring your car back soon after the issue arose, the dealer can say whatever they want and deny this issue is related to the battery... anything could've happened over 3 months, so there's no reason to believe you without evidence

whatever your reasons were, you should've made time or come up with some way and brought the car back right away

but this is what the shop manual says:
View attachment 343087

The OP said the unit stopped working a few days after the battery was replaced. He didn't have time to look into it until recently.