2021 Mercedes-Benz C 300

I am reporting a potentially serious safety problem involving sudden loss of engine power and an unresponsive accelerator. On August 30, 2026, while I was driving on the highway with my child in the vehicle, the engine suddenly stopped producing power. Pressing the accelerator did not restore normal acceleration. The vehicle was in motion, and the loss of power created a serious safety risk. I believe this happened on two occasions, although I do not have the date and mileage for the other occurrence available. The confirmed incident occurred at approximately 38,198 miles. I brought the vehicle to a Mercedes-Benz dealership. The dealership told me the problem was caused by a software defect and that a software update was needed. The update was performed, and I paid for the repair. I am concerned that the vehicle could unexpectedly lose power again while in traffic, putting occupants and other road users at risk. I am asking NHTSA to review this report and assess whether this may indicate a safety-related defect affecting this vehicle or others.

NHTSA ID

11774726

Incident

Aug 30, 2026

The contact owns a 2021 Mercedes-Benz C 300. The contact stated that while depressing the accelerator pedal, there was a loud knocking sound coming from the engine compartment while accelerating. There were no warning lights illuminated. The vehicle was taken to the dealer to be diagnosed, and it was determined that the spawn rod was fractured, causing engine failure. The contact was informed that the engine needed to be replaced. The vehicle was not repaired. The contact stated that the engine was previously repaired because of the head gasket failure in 2024. The manufacturer was informed of the issue, and a case was opened. The failure mileage was approximately 88,000.

NHTSA ID

11755408

Incident

Aug 7, 2024

Mileage

88,000 mi

The contact owns a 2021 Mercedes Benz C. The contact stated that while driving on the highway, the vehicle began to lose power, riding rough. The check engine light illuminated. The vehicle was towed to the dealer where it was diagnosed with a fuel injection failure. The vehicle read diagnostic code P1CEF85. The manufacturer was made aware of the issue. The approximate failure mileage was 60,000.

NHTSA ID

11738727

Incident

Mar 11, 2026

Mileage

60,000 mi

I am reporting a safety defect involving the panoramic sunroof on my 2021Mercedes-Benz Coop vehicle. The panoramic sunroof has malfunctioned due to an apparent design or manufacturing defect. The sunroof fails to open / close and makes grinding or popping noises which malfunctioned unexpectedly. This issue occurred without misuse and during normal operation of the vehicle. The defect creates a safety concern because it can distract the driver, allow water intrusion that may affect electrical components, and/or cause sudden glass or mechanical failure while driving. The vehicle has been taken to a Mercedes-Benz dealership for diagnosis and the issue was not resolved due to denied coverage. I was informed that the repair would be costly despite the issue appearing to be a known defect in Mercedes-Benz panoramic sunroof systems. I believe this defect may affect other Mercedes-Benz vehicles with panoramic sunroofs and should be investigated for a potential safety defect or recall.

NHTSA ID

11712287

Incident

Oct 6, 2025

The contact owns a 2021 Mercedes-Benz C300. The contact stated that while driving at an undisclosed speed, the vehicle stalled and failed to exceed 30 MPH, with the check engine warning light illuminated. The vehicle did not require a restart. The contact also stated that while stopped at a traffic light, the vehicle was shaking abnormally. The vehicle was taken to an independent mechanic, where it was diagnosed and determined that there was a misfire in cylinder #2. The contact was informed that the cylinder head needed to be replaced. The vehicle was not repaired. The vehicle was then taken to the dealer, where the same diagnostic information was provided, and the contact was informed that the cylinder head needed to be replaced. The vehicle was not repaired. The manufacturer was made aware of the failure. The approximate failure mileage was 86,078.

NHTSA ID

11706514

Incident

Dec 22, 2025

Mileage

86,078 mi

2021 Mercedes-Benz C300 with M254 2.0L turbo engine experienced sudden cylinder head failure at 44,000 miles due to manufacturing porosity in the cylinder head and camshaft bore scoring. Dealer confirmed defect. This is a known, recurring issue on 2021–2023 M254 engines with multiple documented failures under 50k miles. Sudden loss of compression and misfires create a safety risk of engine failure or fire while driving. TSB NUMBER LI07.16-P-074000 (February 2023, updated multiple times). This is a known production/early break-in issue with the M254 engine, not the result of owner misuse or delayed maintenance.

NHTSA ID

11701560

Incident

Oct 10, 2025

My car had a coolant leak last year a hose broke and I have a 2021. My son’s mercedes sis the same thing on 11/23/2025. I have researched ma y others with this same issue. There should be a recall on that hose or whatever the problem Is.

NHTSA ID

11701496

Incident

Nov 23, 2025

While driving car on Sunday 10/5, it suddenly failed to respond to gas pedal pressure. The car stopped accelerating and rather slowed to 15 MPH, almost causing a car from behind to hit me in the rear side.

NHTSA ID

11691568

Incident

Oct 5, 2025

Complaint Customer complaints relating to automatic transmission 725.0 (NAG3 Classic /9G‑TRONIC automatic transmission) and 725.1 (NAG3 facelift/9G‑TRONIC automatic transmission facelift) For faster, more effective and more targeted diagnosis, Intelligent Predictive Repair (IPR) has been developed and integrated into XENTRY Diagnosis. This ensures an automated diagnosis process. Cause As part of the on‑going quality monitoring process and associated quality and damaged part appraisals, a high number of ineffective repairs have been discovered. Such ineffective repairs have a negative impact on all those involved. For the customer, it means high costs and long repair times and for the dealer, it may result in the warranty claim being rejected. Remedy General note: IPR is available for: ‑ NAG3 Classic (automatic transmission 725.0) ‑ NAG3 Classic ‑ AMG sport model (AMG 43 and AMG 53 with converter, automatic transmission 725.0) ‑ NAG3 Classic ‑ Hybrid (automatic transmission 725.0) ‑ NAG3 Classic ‑ AMG 63 (automatic transmission 725.0) ‑ NAG3 MOPF ISG2 (automatic transmission 725.1) ‑ NAG3 MOPF Hybrid (automatic transmission 725.1) ‑ NAG3 MOPF AMG sport model (AMG 53/AMG 53e with ISG48V2 & converter /hybrid, automatic transmission 725.1) ‑ NAG3 MOPF AMG 43/63 with engine 139/177 (automatic transmission 725.1) Steps for the Service Advisor: Important: The more information and detail that can be given to the workshop about the complaint, then the more effective the subsequent diagnosis with IPR can be. To this end, a detailed service reception questionnaire has been provided to help the Service Advisor ask the right questions about the complaint. ‑ Ask precise details about the customer complaint and the conditions and context. ‑ Fill out the service reception questionnaire (please see WIS document: OF27.00‑P‑3000‑05A) for customer complaints in detail and forward it to the workshop. Operation steps for the workshop employee: 1. Perform quick test 2. Access the transmission control unit via XENTRY Diagnosis 3. Select "A customer complaint has been received" and carry out all subsequent operation steps (IPR diagnosis path). 4. The IPR repair instructions are displayed, Carry out the IPR repair instructions without performing any further diagnostic or repair steps. ‑ Important: The IPR repair instructions are binding and must be carried out. Differing repairs are possible; such repairs must be documented in XENTRY in free text. Please observe the information regarding the IPR diagnosis process in XENTRY. Conventional diagnosis and subsequent appropriate repairs are only permitted if IPR does not specify any repair instructions. Invoicing for the guided IPR steps: Operation item 27‑0640 and 0.1 hours Notice regarding warranty: The use of IPR in association with warranty claims is mandatory! If the IPR repair instructions are disregarded without justification, the warranty claims may be rejected and/or are subject to debit. IPR will provide a measure to repair the vehicle based on various data sources (EEPROM, fault codes, etc). Please consider the vehicle maintenance record of the vehicle. For warranty and goodwill repairs, the transmission ECU test log and/or IPR test result must be uploaded into Paperless XENTRY Diagnosis (pXD) . If repair is being determined based of oil pan condition pictures of oil pan MUST be provided to warranty as justification. ·Tamper‑proof documentation: The IPR repair instructions are stored in the control unit log. Please file this document in a tamperproof manner ‑ either in digital form or as a hard‑copy printout and also enclose a printout with the damaged part. In this regard, please comply with the country‑specific warranty requirements of the MPC/general distributor.

Complaint Slight creaking noises when steering. Cause Noises may come from one or more sleeves of the horn contact plate. Remedy Remove the horn contact plate and check the sleeves for noise; see attached video. If necessary, apply lubricant to the sleeves.

Complaint Repeat workshop visit for vehicle vibration while braking from 60MPH. NOTE: Although the symptom is similar to a tire or wheel vibration, it is usually readily noticeable that it is distinctly different if it happens at all times or only while braking. Cause Although the cause of the vibration is most likely due to excessive lateral runout on the rotor or hub/wheel bearing, the system as a whole needs to be checked for other possible contributors. 1. Prior collision damage. 2. Excessive vehicle sitting times without rotating components. 3. Wheel bearing/hub lateral runout. 4. Wheel mating interface has been overtightened (did not follow WIS torque procedures). 5. Mating interfaces have been excessively modified (painted, gouged, bent, etc...). Remedy Read through the entire attached worksheet prior to performing. After performing, please review the final page and perform work accordingly. NOTE: This document is not pre‑authorization or otherwise dealing with warranty. This is intended for diagnostic purposes only. If there are any questions on how to proceed OR The work sheet has been performed and the vehicle has returned with the same concern. Please perform the measurements as seen in the worksheet again and open a TIPS case to the Body, Chassis, and Entertainment team.

Complaint Clanking noise from the area of the speakers in the doors. Cause Under analysis. Remedy To narrow down the cause, first of all remove the door trim. • If the noise continues to occur even with the door trim removed, then continue the diagnosis working towards the speaker and if necessary, create a TIPS case with an informative video as an attachment. Note: Please refer to this document in the warranty claim and, once diagnosis is completed, select the damage code depending on the component part that is causing the issue. • If the noise stops when the door trim is removed, please proceed in accordance with LI72.10‑P‑067691. Information: In this case, do not use a damage code for the speakers in the warranty claim!

Complaint Only vehicles with a variable transfer case are affected. This can be identified through control unit N45 or N15/7 in the quick test. When accelerating with light load, vibrations or jolting sensation may come from the drivetrain. It may be mistaken for a harsh shift. Cause Modified oil quality of the transfer case over time Remedy 1. Check initial quick test for faults in the following control units. N45 or N15/7 ‑ All‑wheel drive (ARS) N127 ‑ 'Drivetrain' (PTCU) control unit Y3/8 ‑ Transmission control for 9‑gear transmission (VGS) N3/10 ‑ Engine electronics for combustion engine If faults are present: Process them as per the guided test in Xentry. If no faults are present: 2. Check the transmission oil level If the oil level is correct: 3. Unplug the connector for the transfer cases control unit and reassess the vehicle on a subsequent test drive If the complaint no longer occurs with the control unit disconnected: 4. Perform an oil change of the transfer case according to WIS specifications ‑ Collect an oil sample of the original fluid 5. Complete all transfer case Tech‑in processes located under VG Control Unit > Adaptations ‑ Resetting the oil service data ‑ Calibration of N15/7 5. Perform road test of vehicle to bring transmission to operating temperature (>60C ) 6. Operate vehicle in full circle with steering at full lock ‑ 5x to the left ‑ 5x to the right Return the vehicle to the client and request they drive the vehicle for at least 200 miles and return if the concern is not resolved. The extended drive will ensure the clutches are properly conditioned with the new oil and should resolve the concern. If this LI has already been applied to the vehicle, please submit a TIPS case to the Powertrain inbox with the following information: ‑ Initial quick test with fault freeze frame data ‑ Initial transmission EEPROM data ‑ VGS and Transfer Case Control Unit Log (N45 or N15/7) ‑ Special Acquisition Data .txt file (Transfer Case Control Unit > Special Procedures > Procedures for support queries to market support > Special Data Acquisition) ‑ Photo of the original oil condition ‑ What color was the fluid drained from the transfer case before the oil change? ‑ Was an oil sample collected, and can you provide a photo of the original fluid? ‑ What color and part number fluid was used during the oil change? ‑ How far was the vehicle driven after?

Complaint Customer complaints relating to automatic transmission 725.0 (NAG3 Classic / 9G‑TRONIC automatic transmission) and 725.1 (NAG3 facelift/9G‑TRONIC automatic transmission facelift) For faster, more effective and more targeted diagnosis, Intelligent Predictive Repair (IPR) has been developed and integrated into XENTRY Diagnosis. This ensures an automated diagnosis process. Cause As part of the on‑going quality monitoring process and associated quality and damaged part appraisals, a high number of ineffective repairs have been discovered. Such ineffective repairs have a negative impact on all those involved. For the customer, it means high costs and long repair times and for the dealer, it may result in the warranty claim being rejected. Remedy General note: IPR is available for: NAG3 Classic (automatic transmission 725.0) NAG3 ‑ AMG sports models (AMG 43 and AMG 53 with converter) NAG3 ‑ AMG 63 NAG3 ‑ Hybrid NAG3 Facelift (automatic transmission 725.1) NAG3 facelift (725.1) with engine 139/177 (AMG 43/63) Steps for the Service Advisor: Important: The more information and detail that can be given to the workshop about the complaint, then the more effective the subsequent diagnosis with IPR can be. To this end, a detailed service reception questionnaire has been provided to help the Service Advisor ask the right questions about the complaint. ·Ask precise details about the customer complaint and the conditions and context. ·Fill out the service reception questionnaire (please see WIS document: OF27. 00‑P‑3000‑05A) for customer complaints in detail and forward it to the workshop. Operation steps for the workshop employee: Perform quick test ·Access the transmission control unit via XENTRY Diagnosis ·Select "A customer complaint has been received" and carry out all subsequent operation steps (IPR diagnosis path). ·The IPR repair instructions are displayed. Carry out the IPR repair instructions without performing any further diagnostic or repair steps. ·Important: The IPR repair instructions are binding and must be carried out. Differing repairs are possible; such repairs must be documented in XENTRY in free text. Please observe the information regarding the IPR diagnosis process in XENTRY. ·Conventional diagnosis and subsequent appropriate repairs are only permitted if IPR does not specify any repair instructions. ·Invoicing for the guided IPR steps: Operation item 27‑0640 and 0.1 hours !!!!! Notice regarding warranty: !!!!! ·The use of IPR in association with warranty claims is mandatory! ·If the IPR repair instructions are disregarded without justification, the warranty claims may be rejected and/or are subject to debit. IPR will provide a measure to repair the vehicle based on various data sources (EEPROM, fault codes, etc). Please consider the vehicle maintenance record of the vehicle. For warranty and goodwill repairs, the transmission ECU test log and/or IPR test result must be uploaded into Paperless XENTRY Diagnosis (pXD) . If repair is being determined based of oil pan condition pictures of oil pan MUST be provided to warranty as justification. ·Tamper‑proof documentation: The IPR repair instructions are stored in the control unit log. Please file this document in a tamperproof manner ‑ either in digital form or as a hard‑copy printout and also enclose a printout with the damaged part. In this regard, please comply with the country‑specific warranty requirements of the MPC/general distributor.