My GMC Hummer EV says "Service High Voltage System." What do I do?

Short answer: Start with the 12-volt battery: owners say a weak or loose one is behind many Hummer EV warnings, and a 12-volt reset cleared it for several. If the warning comes back, or the truck won't charge, book the dealer and ask them to find the cause, not just clear the code. Owners have had chargers, charging modules and battery cells replaced under warranty. Press the OnStar button for a diagnostic report to bring with you.

Symptom: A 'Service High Voltage System' message, sometimes with a high-voltage battery fault, 'Unable to charge' or a warning that the vehicle won't restart.

What to check first

1. Check that the 12-volt battery terminals are tight and have the battery load-tested.
2. If you're comfortable, do the 12-volt reset in the owner's manual. Never touch the orange cables.
3. Request an OnStar diagnostic and save the report.
4. If the warning returns or charging stops, book the dealer and ask for the codes in writing.
5. Run your VIN at nhtsa.gov/recalls for the high-voltage battery recalls.

Likely causes

1. Weak, dying or loose 12-volt battery. Almost every modern car, including EVs, has a small 12-volt battery that powers the computers, locks and screens. When it's weak or isn't being recharged, the car can show a pile of unrelated warnings or refuse to go into gear, even with a full main battery.
2. Software fault. Some owners' warnings went away after the dealer installed software updates.
3. Charging hardware or battery cooling. Owners had a high-voltage charger and a charging module replaced. GM's own message says the warning can point to a battery cooling system problem.
4. High-voltage battery cell or pack fault. The big battery is made of many cells. One owner waited weeks for a cell replacement.

What it usually costs

Owners' charger, module and battery-cell repairs were done under warranty and they didn't post prices.

Is it safe to keep driving?

If the truck drives normally, owners drove to the dealer. If it says it won't restart, shows a red warning, won't charge, or you smell burning, stop driving and call GMC roadside.

DIY or shop?

Checking and resetting the 12-volt is DIY. Everything on the high-voltage side is dealer work. An EV's high-voltage system can stay live with the car off. Leave the orange cables, the big battery and the drive unit to a trained EV technician.

Recalls, bulletins and complaints in plain words

• Related recalls: NHTSA lists 22V771000 (some 2022-2023 Hummer EV Pickups: the high-voltage battery pack may not be sealed, letting water in), 23V367000 (some 2022 Hummer EVs: connections inside the high-voltage battery may be out of position or badly welded, which can cause a loss of drive power), 24V320000 (some 2024 Hummer EV SUVs: wires in the rear drive motor may not be insulated well enough, which can cause a loss of drive power) and 26V494000 (some 2022 Hummer EV Pickups: a part inside the high-voltage battery may not be secured properly). Check your VIN at nhtsa.gov/recalls. If your car is part of a recall, the dealer fixes it for free.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect.
• NHTSA complaints: 11 keyword-matched complaints for GMC Hummer EV Pickup / SUV 2022–2025 (of 33 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-31; keywords: high voltage, 12 v, battery, charg, won t start, dead, propulsion) (NHTSA returned no data for 14 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In Had a ton of serious warnings including High Voltage Battery Fault, Service High Voltage System. Solved by a 12V reset. on Reddit r/HummerEV, an owner found a pile of warnings, including Service High Voltage System and a high-voltage battery fault saying the truck wouldn't restart. A 12-volt reset fixed it, and they suspected a loose negative terminal. Another owner said most Hummer EV issues they'd seen trace to a low or dying 12-volt battery, and one said the dealer fixed theirs with updates.
• In Service high voltage system on Reddit r/HummerEV, a new 2025 SUV owner got the warning on the highway. Another owner said the first time, the dealer just cleared the code; the second time the truck sat a week while they replaced the high-voltage charger and a charging control module, and they warned not to let the dealer just clear the code.
• In Went to drive today , only to find Error… on Reddit r/HummerEV, an owner who got the warning heard from others that disconnecting and reconnecting the 12-volt cleared it for one at about 900 miles, while another's new 2025 sat four weeks at the dealer for a battery cell replacement. One suggested pressing the OnStar button for a remote diagnostic to hand to the dealer.
• In Service High Voltage Warning Message on Hummer EV Forum, an owner got the warning two months in. The truck drove fine but wouldn't charge. They posted GM's guidance that the message can mean a battery cooling problem and said that guidance didn't fix theirs.

Quick tip

A 12-volt reset is a fine first step, but if the message comes back, don't keep doing it. Make the dealer find the cause and write it down while you're under warranty.

Vehicle focus: GMC Hummer EV Pickup and SUV, 2022-2025.

Safety / mechanic note

High-voltage battery faults are a safety item. Verify with a mechanic.