Why does my Kia Niro Plug-in Hybrid keep killing its 12V battery, or run the engine in EV mode?

Short answer: If your Niro Plug-in Hybrid goes completely dead, it's almost always the 12V battery in the trunk. The PHEV has no reset button like the regular hybrid, so jump it, then have the battery tested and look for anything plugged into the OBD port. If the gas engine runs in EV mode, check whether climate control or defrost is asking for heat.

Symptom: Nothing turns on, the car won't start charging when plugged in, the 12V keeps going flat, or the gas engine runs while the car says EV mode.

What to check first

1. Jump the 12V (it's in the trunk on the right side, under a cover).
2. Have the 12V load-tested. A load test puts a real demand on the battery. A plain voltmeter reading can look fine on a battery that can't deliver power.
3. Look for a GPS tracker or dongle in the OBD port under the dash.
4. Turn climate control and defrost off and see if the engine stops running in EV mode.
5. Run your VIN at nhtsa.gov/recalls.

Likely causes

1. Weak or drained 12V battery. Unlike the regular Niro hybrid, the plug-in uses a normal 12V battery in the trunk with no reset button. Once it's been run flat a few times it may need replacing. The 12V battery is the small battery that powers the computers, locks and screens. EVs and plug-in hybrids still need it to wake up and connect the big battery.
2. A plug-in GPS tracker or OBD device. One owner found a hidden third-party GPS tracker plugged into the OBD port that kept draining the 12V. The dealer never found it.
3. Cabin heat or defrost demand (engine runs in EV mode). Owners said the gas engine starts in EV mode to make heat when climate control or the defroster asks for it, on models without a heat pump. It isn't a fault.

What it usually costs

Owners had the 12V replaced for free under warranty. The threads didn't post out-of-warranty prices.

Is it safe to keep driving?

Yes, once it starts. A dead 12V can strand you, so carry a jump pack until it's fixed. The engine running for heat is normal.

DIY or shop?

Jumping, testing and replacing the 12V and removing OBD devices are DIY. Hybrid system faults are a dealer job.

Recalls, bulletins and complaints in plain words

• Recalls: NHTSA lists recall 23V534000 for certain 2018-2022 Niro Plug-in Hybrids: fluid can leak into the circuit board in the hydraulic clutch actuator, which can cause an engine compartment fire. It isn't a 12V battery recall, but 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: 20 keyword-matched complaints for Kia Niro Plug-in Hybrid 2018–2022 (of 59 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-04; keywords: ev mode, electric mode, engine (turns, kicks, comes) on, gas engine, hybrid, plug in, phev, 12v, 12 volt, battery). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In No 12 volt reset switch on 23 Nero plug in hybrid on Reddit r/KiaNiro, a 2023 Niro PHEV owner couldn't get anything to turn on, not even the dash, and found no 12V reset button. Replies said only the regular hybrid has that button. The PHEV's 12V is in the trunk under a cover and needed a jump. One owner's dealer replaced the 12V for free, and another found a hidden GPS tracker in the OBD port draining it.
• In Kia Niro PHEV 12 volt battery dies on Reddit r/electricvehicles, a 2024 Niro PHEV owner's 12V kept dying. Replies suggested a battery tender and pushing for warranty replacement. A 2019 Niro PHEV owner said the original 12V was still fine after 4 years of mostly EV driving.
• In 2023 PHEV occasionally gets stuck in doing RPMs when supposedly in EV mode, even when parked. Any ideas on issue? (More info in comments) on Reddit r/KiaNiro, a 2023 Niro PHEV owner's gas engine ran at about 1,300 RPM while in EV mode, even parked. Other owners said it starts for cabin heat or defrost when climate control is on, and it stopped when they turned climate control off.

Quick tip

Carry a jump pack, check the OBD port, and get a weak 12V replaced under warranty while you can.

Vehicle focus: Kia Niro Plug-in Hybrid, 2018-2022 (owner reports also cover the 2023-2024 model).

Safety / mechanic note

An EV's or plug-in hybrid's high-voltage system can stay live with the car off. Leave the orange cables, the big battery and the charging hardware to a trained technician. Verify with a mechanic.