Why does my BMW i3 REx say 'Drivetrain malfunction' and the range extender won't run?

Short answer: On the i3 REx, 'Drivetrain malfunction' with the range extender shutting off is usually a misfire. Owners fixed it with new spark plugs or coils, fresh gas, or a $20 fuel pump relay. Overfilling the tank can also trip the fuel tank pressure sensor. Read the codes first.

Symptom: A 'Drivetrain malfunction' message, a check engine light, the range extender won't start or shuts off, or the car says to continue in electric-only mode.

What to check first

1. Read the codes with an OBD2 reader and a BMW app.
2. If you see misfire codes, check spark plugs and coils.
3. Use fresh gas. If you rarely use the REx, burn through old fuel.
4. Carry or swap a spare fuel pump relay.
5. Don't overfill the tank, and check the gas cap.
6. Run your VIN at nhtsa.gov/recalls for 17V088000.

Likely causes

1. Misfire from worn spark plugs or coils. The range extender (REx) is a small gas engine that charges the battery. Owners said misfires set 'Drivetrain malfunction' and shut it down, and one owner fixed it with new spark plugs.
2. Old, wrong or bad gas. The REx often sits unused, so gas goes stale. Owners fixed misfires with fresh gas and a fuel additive, and some at high altitude said lower octane worked better.
3. Fuel pump relay. Owners said the fuel pump relay (about $20) fails often and stops the REx from starting.
4. Fuel tank pressure sensor or overfilling. Owners said overfilling the tank can flood the pressure sensor and set a code, and one 2014 owner's lean misfire codes pointed to that sensor, which is part of the tank assembly.

What it usually costs

A fuel pump relay is about $20. Owners replaced spark plugs themselves. In California, owners said emissions parts are covered for 15 years/150,000 miles.

Is it safe to keep driving?

Mostly yes, on electric power. The REx shutting off can strand you if the battery runs low, so plan charging. If you smell fuel, stop and get it checked.

DIY or shop?

Reading codes, plugs and relays are DIY. Fuel tank sensor and engine repairs are shop jobs.

Recalls, bulletins and complaints in plain words

• Related recalls: NHTSA lists 17V088000 (2014-2017 i3 REx: the fuel tank vent line may rub against a battery cable and leak fuel vapor), and 19V310000 and 19V599000 (2018 and 2019 i3: a circuit board in the electric motor electronics may cut high-voltage power). If your car is part of a recall, the dealer fixes it for free. Check your VIN at nhtsa.gov/recalls.
• 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 files the REx under the i3, and some queries returned no data.
• NHTSA complaints: 46 keyword-matched complaints for BMW i3 (all, including REx) 2014–2021 (of 182 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-03; keywords: range extender, rex, engine, drivetrain, drive train, loss of power, lost power, reduced power, misfire, check engine) (NHTSA returned no data for 6 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In 2016 REx: Desperately trying to resolve Drivetrain Errors/Misfires Cylinders 1 & 2 (Advice requested!) on Reddit r/BMWi3, a 2016 REx owner got drivetrain errors and misfires on cylinders 1 and 2 after a fill-up. Replies suggested bad gas, a fuel tank pressure sensor (one 2014 owner's lean misfire came from it), and lower octane at altitude. The owner planned to drain the tank, add injector cleaner and do a few oil changes.
• In Engine Check light then Drivetrain errors? 2015 Rex version, thought the codes were related to some sensor and went for a 50 miles drive when the Drivetrain error came up and the range extender won't start.. on Reddit r/BMWi3, a 2015 REx owner got a check engine light and then a drivetrain error, and the REx wouldn't start. Replies pointed to plugs and coils, said owners should carry a spare fuel pump relay (about $20) because it fails often, and warned that repeated misfires can be the first sign of bigger engine trouble.
• In Drive train malfunction range extender on Reddit r/BMWi3, an owner's drivetrain malfunction kept shutting down the REx. A new 12V battery and fuel pump relay didn't fix it. The owner later replaced the spark plugs and the errors stopped. Others said premium fuel at high altitude in the cold caused the same error, and mid-grade there helped.
• In Check engine light on - I have 54k mikes and it’s a 2016 i3 with range extender. Worried may cost a lot - does anyone know what may be happening here? on Reddit r/BMWi3, a 2016 i3 REx owner at 54,000 miles got a check engine light. Replies said overfilling the tank can flood the pressure sensor, a loose gas cap can set it, and cold weather brought on misfire codes. Owners in California and other CARB states said emissions parts are covered by a 15-year/150,000-mile warranty there.

Quick tip

Read the codes before paying for a dealer diagnosis. Plugs, gas and the relay fix most of these.

Vehicle focus: BMW i3 with range extender (REx), 2014-2021.

Safety / mechanic note

Fuel leaks are a fire risk. An EV'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 EV technician. Verify with a mechanic.