BMW bulletin B340921
What it saysA turbine-like noise can be heard from the rear axle while deceleration during the first 300 miles of operation when braking from low speed. NHTSA’s full summary ↓
BMW filed this with NHTSA as a service bulletin dated Jan 14, 2025, received Nov 8, 2021, about the service brakes — 3 documents under this number. It names 7 model years (2016–2022). A service bulletin is not a safety recall: it is the manufacturer telling its dealers how to diagnose or repair a condition, and whether the work is paid for depends on the warranty in force. Quoting the number at a service desk can save a diagnosis.
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What the bulletin says
Source: NHTSA's summary of the manufacturer's communicationA turbine-like noise can be heard from the rear axle while deceleration during the first 300 miles of operation when braking from low speed.
Documents filed under this number
| Document | Kind | Dated | NHTSA id | At NHTSA |
|---|---|---|---|---|
| B340921 | Service bulletin | Jan 14, 2025 | 11013041 | PDF, 115 KB |
| B340921 | Service bulletin | Feb 1, 2023 | 10233158 | not found |
| B340921 | Service bulletin | Sep 1, 2021 | 10203335 | not found |
The summary is NHTSA’s abstract of the document, as filed under 49 CFR 579.5. One communication is usually filed as several documents — the bulletin, a dealer notice, an owner letter — and the number on any of them finds this page. A PDF link is shown only once NHTSA’s own server has been seen to serve that document; the rest are checked over the following nights. Quote the number at a service desk; it is what the dealer’s system is keyed on.
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Model years it names
Source: as the manufacturer filed themA vehicle page’s bulletin list shows the newest few communications filed against that model year; the count there is every one.
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What owners reported
Source: NHTSA complaint databaseOwners of the 7 model years this bulletin names have filed 215 complaints with NHTSA since 2015. 28 of them name Power Train, Service Brakes — the same broad component group this bulletin is filed under. NHTSA groups complaints coarsely, so that means the same area of the vehicle, not necessarily the same fault.
The contact owns a 2016 BMW M4. The contact stated that under previous ownership, the vehicle was previously repaired under NHTSA Campaign: 15V782000 (Power Train) at a local dealer. The contact stated that two years later, the contact was unsure if the vehicle was included or repaired under NHTSA Campaign: 18V713000 (Power Train).
2016 BMW M · Power Train · filed with NHTSA Dec 8, 2025 · ODI 11703660
Fuel pump failed within a minute of initial warning and performance indicating a problem. While driving ~60 mph on the highway, available power dropped to zero within a couple tenths of a mile, *barely* allowing me to move to the shoulder. Screen error/fault indicated vehicle was ok to keep driving but to schedule service soon.
2020 BMW M · Power Train · filed with NHTSA Jul 14, 2025 · ODI 11673452
BMW dealer noted "clunk on accel" at 57,152 miles on RO #224018 under engine mounts. Dealer quoted $3,463.38 for engine mounts. I declined repair. Clunk on acceleration is still present at 60,700 miles. Same symptom as BMW Service Bulletin SIB 27-02-20 for 4 Series Drive jerking/shuddering from transfer case.
2021 BMW M · Power Train · filed with NHTSA Oct 15, 2026 · ODI 11771285
A complaint is one owner’s account, not a defect finding, and a bulletin is not a recall. Nothing here says this bulletin was issued because of these complaints, or that the repair it describes fixed them. Complaint files on this site begin in 2015.
More bulletins about the service brakes
Source: same model, same component group6 other manufacturer communications filed against the BMW M under the same component group (service brakes), newest first.
| Number | Kind | Dated | NHTSA’s summary |
|---|---|---|---|
| B340925 | Service bulletin | Dec 17, 2025 | There may be an internal leakage in the brake fluid expansion tank (aka brake fluid reservoir). In the rarecase of a leak, the filling level… |
| B064725 | Other | Sep 9, 2025 | Protection Plate Return |
| 0034200300 | Service campaign | Jul 30, 2025 | There may be an internal leakage in the brake fluid expansion tank (aka brake fluid reservoir). In the rare case of a leak, the filling leve… |
| B341025 | Service bulletin | Jul 14, 2025 | After programming from the I-Levels between …24-03-505 and lower than …24-03-540 to I-Level …24-11-530 or higher, the following CCM may be d… |
| B340625 | Service bulletin | May 2, 2025 | Brake “groaning” noises heard at the rear axle at slow speeds, i.e. close to standstill, stop and go driving;when parking or maneuvering. |
| B340714 | Service bulletin | Oct 13, 2023 | "Carbon Ceramic Brake Noise. INFORMATION ""M"" vehicles are designed for high-performance, spirited driving. To accommodate this type of dri… |
Quote the number at the service desk. It is what the dealer’s system is keyed on.
Every manufacturer communication NHTSA holds for BMW, by the number printed on the letter.
Common questions
Source: NHTSA, 49 CFR 579.5Is BMW B340921 a recall?
No. A service bulletin is a manufacturer communication filed with NHTSA under 49 CFR 579.5; a safety recall carries an NHTSA campaign number (two digits, a letter, six digits) and a federal duty to repair for free. Check the vehicle’s recalls by VIN at nhtsa.gov/recalls, or on its page here.
Is the B340921 repair free?
Not automatically. A service bulletin describes how to diagnose or repair a condition; who pays depends on the warranty in force, any extension, and the dealer. Quoting the number can save a diagnosis either way. Every BMW warranty extension on file lists the ones BMW has filed.
Which vehicles does B340921 cover?
The manufacturer filed it against 7 model years: 2022 BMW M, 2021 BMW M, 2020 BMW M, 2019 BMW M, 2018 BMW M, 2017 BMW M and more. Whether one particular vehicle is affected is a matter of its VIN and its build, not its model year alone.