Why does my BMW i3 say drivetrain malfunction or power reduced?

Short answer: On the i3, a 'drivetrain malfunction' warning is a catch-all message. Owners most often trace it to a weak 12-volt battery, moisture or corrosion in the high-voltage cables, or, on REx cars, the range extender (the small gas engine that makes electricity) and its electronics overheating. Start by testing the 12-volt battery and reading the codes with a BMW-capable scanner. Repeat warnings while the REx runs need a shop that knows the i3.

Symptom: A yellow or red 'drivetrain malfunction' message, a check engine light, reduced power, or the range extender not starting. Some cars only show it after long drives on the REx.

What to check first

1. Test the 12-volt battery. An old one can set many warnings. Owners note it isn't always the cause, so read codes too.
2. Read the codes with a BMW-capable app or scanner (owners use BimmerLink). Write down any range extender (REME) codes.
3. Note when it happens. If it only shows after a long time on the REx, tell the shop. That points to the REx side.
4. Check your VIN for recalls, including 19V310 (2018) and 17V088 (REx fuel vent line).
5. If the car went through deep water, mention it. Owners linked some faults to moisture in high-voltage connections.

Likely causes

1. Weak or failing 12-volt battery. Low voltage makes the car's computers set fault codes.
2. Range extender electronics overheating (REME inverter). Owners see code 21DDD0 for power reduced from high temperature.
3. Corroded or damaged high-voltage cable, or a leaking heater pump. Owners reported revised cables and a leaking heater pump fixing repeat errors.
4. Faulty motor electronics (EME) module. Recall 19V310 replaced it on some 2018 cars.

What it usually costs

Costs vary a lot. A 12-volt battery is the cheap fix. One owner on r/BMWi3 said a heater pump that leaked onto a high-voltage cable meant replacing both, about a $3,000 job, which their extended warranty paid. Another owner said corroded high-voltage cables to the REx made the repair too expensive to keep the car.

Is it safe to keep driving?

Pull over safely if you lose power. Many owners keep driving on battery when the REx has a fault, but a drivetrain warning can also come before the car shuts down power. Get it diagnosed soon and don't count on the REx for a long trip until it's fixed. Verify with a mechanic.

DIY or shop?

DIY for testing or replacing the 12-volt battery and reading codes. Anything high voltage is shop or dealer work. Don't open orange high-voltage cables yourself.

Recalls, bulletins and complaints in plain words

• Recall 19V310 (2018 i3 and i3 REx): a circuit board in the electric motor electronics (EME) could shut off high-voltage power, so the car loses drive power. If your car is part of this recall, the dealer replaces the module for free.
• Recall 17V088 (2014–2017 i3 REx): the fuel tank vent line could rub on the battery cable and leak fuel vapor. If your car is part of this recall, the dealer inspects it and adds a clip for free.
• NHTSA complaints: 58 keyword-matched complaints for BMW i3 and i3 REx 2014–2018 (of 218 unique complaints queried, NHTSA complaints API by model year, queried 2026-10-21; keywords: range extender, REx, reduced power, drivetrain, loss of power, stall, shutdown) (NHTSA returned no complaint data for 14 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In Check engine light and weird errors on r/BMWi3, a 2015 REx owner had a check engine light a month after a new 12-volt battery, with codes for the range extender electronics and '21DDD0 REME inverter, power-reduced operation: temperature threshold exceeded'. Another owner had the same codes after driving through high water and planned to check the high-voltage cables for moisture. Others suggested proper BMW diagnosis.
• In [Drivetrain error [2016 REX]](https://www.reddit.com/r/BMWi3/comments/1aiqqay/drivetrain_error_2016_rex/) on r/BMWi3, owners said a new 12-volt battery fixed some odd faults but not always the drivetrain error. One said corroded high-voltage wires between the REx and battery caused theirs. Another said BMW knows of a revised high-voltage cable and a heater pump that can leak onto a cable, which cost about $3,000 to fix.
• In drivetrain malfunction on the MyBMWi3 forum, members explained that the REx just runs a generator, so many i3s keep running as battery-only cars when it fails. They said an old 12-volt battery can drop voltage and block the car from starting. One 2015 REx owner said their drivetrain warning turned out to be a damaged A/C condenser leaking after a rock hit it.
• In Check engine light and REX issues on r/BMWi3, a 2019 REx owner had repeat check engine lights for fuel in the charcoal canister, even though they stopped filling at the first click, and also saw the 21DDD0 code.

Quick tip

With an i3, a good code reader app pays for itself. Know your codes before you walk into a shop.

Vehicle focus: 2014–2018 BMW i3 and i3 REx (range extender).

Safety / mechanic note

A drivetrain fault can reduce or cut power while driving. Verify with a mechanic.