HV Battery Shut Off - MG4 suddenly died

[HRL]

Yesterday had a weird experience with MG4 Trophy, just under 3 years old. Left on school run as usual and after about 300 yards it suddenly died, no power at all, blocking a road and wouldn't allow being pushed forwards or backwards. Tried a few times to lock/unlock but no change, message on dash was 'HV Battery Shut Off'. Rang MG garage (no help), rang MG Assist and whilst on phone to them got back into car again and messages all gone and it worked as usual and I just drove home again and not dared go out since! Nothing came up on the app (unlike the constant tyre pressure warnings I get in cold weather).
So did it just have a moment and I should ignore and assume all OK now? Or should I take to garage and potentially have to leave it for weeks to be checked? Can garage look at diagnostics online without me going there? Anyone else had this?
Thanks in advance.

 

[ReintjeWA]

I don’t think the dealer can check diagnostics remotely. I would get them to look into it if only to ease your mind a bit.

I had a conrod escape through the side of the block in the RCZ once, even though the replacement engine was newer it made me hesitant to drive for a good while after. Something like what happened to you can really take the joy out of a vehicle.

 

[ReintjeWA]

Also might want to check out this thread (not sure if it is exactly the same as what happened to you)

E

Thread 'total loss of power on a busy dual carriage way - I'm really scared it will happen again'

Jul 19, 2025

My MG4 Trophy lost all power whilst I was accelerating to get on to a busy dual carriage way on Thursday. It was a hugely scary experience for me & my child as we tried our best to carry out all the correct actions & report the obstruction to emergency services whilst barely being able to hear the operator on the other end of the phone due to the traffic noise. Most drivers seemed to take no notice of my arm waving to slow them, or my red triangle. I sit here now simply relieved that no one was hurt or killed during this incident. Up until now I've loved my MG4 (despite the odd niggle) but...

• Ezza
• carriageway diagnostics emergency services mg4 scared total power loss
• Replies: 35
• Forum: MG4 EV Forum

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[HRL]

Update....so tried car again today, 2 days after first breakdown. Seemed ok and did a few short trips, then suddenly got same messages flash up this evening, complete loss of power again, left it for 10 mins and tried again still no good. Fortunately was in Tesco car park so safe place to leave it and got MG Assist coming tomorrow morning. They assured me that even if it seems to work tomorrow they will follow me to MG Garage. Saying that garage say can't look at it until end of the month, and courtesy car only for 3 days .....mmmmmmmm.
Will post again once got info back from garage as might help others in future.

 

[HRL]

Update ....recovered to MG garage. They actually managed to look at it within 3 days. Did 6 software updates (interesting so many since service in April), and said fine to go....fingers crossed everything ok now !

 

[tsedge]

Glad you got a quick response.

Giving you the latest software updates makes sense, since that can be the problem.

If it is a hardware fault you will know it now if it happens again.

 

[Everest]

Did that say exactly what the fault was or what they found having put their diagnostic system on it?

Or did they not find anything and just update the software "just in case"?

 

[HRL]

Did that say exactly what the fault was or what they found having put their diagnostic system on it?

Or did they not find anything and just update the software "just in case"?

No, literally just told they did 6 updates. I asked about diagnostics, and if they could see that full power lost twice, and what might have been trigger ....and was told they were instructed just to do the updates ?! Don't think any hardware was checked. Keeping everything crossed !

 

[tsedge]

Sounds like "update software and hope". I wouldn't be too confident this fix will work.

 

[Everest]

No, literally just told they did 6 updates. I asked about diagnostics, and if they could see that full power lost twice, and what might have been trigger ....and was told they were instructed just to do the updates

Fingers crossed for you, but IME, from my engineering background, that doesn't sound like a satisfactory resolution of an unknown problem.

If there was an issue with your previous software then, presumably, many others would have experienced the same, which they haven't?

One can only hope that there was a specific issue with your vehicle that has had a software workaround done to it - but poor show IMHO for no feedback on the cause and some clarity of how a software updated fixes it

Hope it now behaves - do keep us updated.

 

[tsedge]

I agree although it seems standard practice with MG to "try software updates first".

It is 2026, where are:

- Remote diagnostics (no need to go to dealer)

- Automatic over-the-air updates (everyone on the latest software)

- AI-assisted fault detection software (for the technicians), learning from other examples and spreading knowledge instantly

???

 

[T1 Terry]

I agree although it seems standard practice with MG to "try software updates first".

It is 2026, where are:

- Remote diagnostics (no need to go to dealer)

- Automatic over-the-air updates (everyone on the latest software)

- AI-assisted fault detection software (for the technicians), learning from other examples and spreading knowledge instantly

???

If you are going to smoke that stuff and post on the forum, the least you could do is PM some to me

We are talking about an MG4 here, not one of the IM range. I have my doubts there is an actual working diagnostic system in an MG4, just fault codes I'd imagine.

There are a lot of things that will disable the HV battery connection, any where from a loose plug/connection, to a dodgy contactor and even the crash module.

I hope the software updates actually fix it, but like others have said, I have my doubts that was the problem.

It might require someone who will actually take the time to fault find the problem, I was going to recommend the workshop that found the no heater problem where the other didn't seem to be able to ..... but that was in Germany ... might be a long drive

T1 Terry

 

[tsedge]

If you are going to smoke that stuff and post on the forum, the least you could do is PM some to me

Just giving MG some gentle encouragement!

We are talking about an MG4 here, not one of the IM range. I have my doubts there is an actual working diagnostic system in an MG4, just fault codes I'd imagine.

They do have a UK diagnostic system, however it seems that full access to information is China only and a lot of it is "raise a ticket with China" and see what they say with each car (unless the garage has seen it before).

This is a stupid and rubbish way to work and needs changing.

There are a lot of things that will disable the HV battery connection, any where from a loose plug/connection, to a dodgy contactor and even the crash module.

Agreed, which is why some AI-assisted diagnostics would be so useful to pattern match similar cases.

 

[LenEV2016]

Just giving MG some gentle encouragement!

They do have a UK diagnostic system, however it seems that full access to information is China only and a lot of it is "raise a ticket with China" and see what they say with each car (unless the garage has seen it before).

This is a stupid and rubbish way to work and needs changing.

Agreed, which is why some AI-assisted diagnostics would be so useful to pattern match similar cases.

It also just makes the whole process drag on a lot longer than it really should do