Hey everyone, I could really use some help diagnosing my 2016 BMW 228i Convertible (F23, automatic, approximately 77,300 miles).
Here’s what happened:
Yesterday I was stopped at a red light with my foot on the brake, seat belt fastened, and driver’s door closed. When the light turned green, I pressed the accelerator, but the engine just revved without the car moving.
I noticed the transmission had switched into Park automatically without me selecting it. I put it back into Drive, got a transmission malfunction warning, and drove about 0.2 miles to park safely. I ran bimmerlink to see what’s wrong and there were no fault codes but it said that a lot of my systems were undervolted.
After restarting the car, the warning disappeared. The transmission shifted normally afterward, including in Sport mode using the paddle shifters to test.
I drove to the closest O’Reillys and bought a brand new battery
The original BMW AGM battery was approximately 10 years old. I replaced it with a brand-new Super Start Platinum AGM Group 49/H8 (95Ah, 900 CCA) from O’Reilly.
I registered the replacement using BimmerLink. The car is coded for a 90Ah AGM battery.
However, immediately after installing the new battery, the car wouldn’t start until someone helped jump-start it.
Afterward, I drove to McDonald’s, turned the car completely off, ate inside, and restarted it successfully without a jump.
Later that evening, the car refused to start again.
Current symptoms:
“Battery discharged. Start engine” warning
“Increased battery discharge” warning
Occasional “Starter/ignition” warning
Clicking/buzzing when attempting to start
Sometimes the engine cranks normally, sometimes it won’t crank at all
Dashboard lights and electronics still work
Car occasionally starts normally after sitting
Chassis stabilization warning appeared
Clock/date settings were reset
After sitting another 6–7 hours, the car still had intermittent starting problems.
Relevant BimmerLink codes:
420571 – EGS power supply below 6.5V
213A21 – On-board power supply voltage too low
802690 – Gear selector release button stuck
802696 – Gear selector undervoltage
There are multiple additional undervoltage codes across other modules.
I haven’t cleared the fault memory or reset transmission adaptations.
BimmerLink previously reported 97% battery charge, but I haven’t measured actual voltage or had the new battery load-tested.
The battery terminals have anti-corrosion grease applied by the O’Reilly employee who helped install the battery. The connections look secure, but I haven’t verified them electrically.
My questions:
Could a faulty engine ground strap, loose battery connection, or IBS issue cause all these symptoms?
Could the voltage problems explain the unexpected Park engagement at the red light?
Should I investigate the starter/solenoid or battery connections first?
Has anyone experienced something similar after replacing and registering a BMW AGM battery?
What tests would you recommend before I start replacing more parts?
The car is currently parked. I’m trying to avoid throwing money at random parts without properly diagnosing it.
Any advice from BMW owners or technicians would be appreciated. Thank you so much I would really appreciate the help :)