Diagnostic article Generic OBD-II
P0944: Mercedes auxiliary transmission-pump fault and the converter-housing check valve
P0944 is the fault addressed in this Mercedes transmission-control case, which reports an internal fault of electric auxiliary oil pump M42 and unavailable ECO start/stop.
Read before applying this article
Documented application
Mercedes-Benz LI27.57-P-053496, version 3, September 2021: NAG2 FEPlus 722.9 transmission with B03 auxiliary oil pump, through transmission sequence 7.248.200. Confirm the installed transmission identification; the source is not a blanket model-year application.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
A pump-labelled fault can involve hydraulic leakage
P0944 is the fault addressed in this Mercedes transmission-control case, which reports an internal fault of electric auxiliary oil pump M42 and unavailable ECO start/stop. The documented cause is leakage between the auxiliary pump and torque-converter housing. The check valve in the housing’s pressure duct is therefore a central diagnostic item, rather than assuming that a pump-related fault proves an isolated electric-motor failure.
An absent fault still has a defined diagnostic order
When the fault is no longer present, the document starts with the auxiliary-pump connector and fuse, then supply voltage and CAN wiring. It next checks transmission-controller software before inspecting the housing check valve. Normally, a step that resolves the complaint ends the sequence; the explicit exception is that the valve inspection still follows the software step. This exception matters when a software update appears to restore stop/start temporarily.
The valve result changes the replacement scope
If the check valve is defective, the manufacturer calls for replacing both the converter housing and auxiliary pump. Pump replacement alone is the later branch in the listed sequence, not the remedy for a failed housing valve. After testing or repair, the transmission oil level must be checked and corrected. Ask the workshop to document the transmission sequence, valve finding and final oil-level verification. This article explains the decision path; it does not supply a universal oil-fill temperature or a safe owner procedure for opening the transmission.
Source and applicability
Mercedes-Benz LI27.57-P-053496, version 3, September 2021: NAG2 FEPlus 722.9 transmission with B03 auxiliary oil pump, through transmission sequence 7.248.200. Confirm the installed transmission identification; the source is not a blanket model-year application.
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Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Hydraulic Pressure Unit Loss of Pressure
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT