Mercedes restraint modules watch their own supply voltage, and B1003 is the module reporting that terminal 15R and terminal 15 came in below what it needs. The system disables itself rather than run on a supply it cannot trust.
Terminal 15 is the switched ignition feed in Mercedes wiring nomenclature, and 15R is the accessory related branch of it. The restraint systems control unit needs both to sit at a healthy level, because firing an airbag takes current and a module running on a sagging supply cannot guarantee it. B1003 is the entry made when either feed reads undervoltage.
Undervoltage codes point outward more often than inward. The module is usually fine and is telling you something about the charging system, the battery, a fuse panel connection, or a ground. On these platforms a weak battery, a corroded ground strap, or a tired connection at the fuse and relay box will set restraint codes long before the car shows any other symptom.
B1003 reports a live measurement, so it belongs with the circuit codes and it comes and goes with the actual supply voltage. It can also set once during a low voltage event and stay stored as a history entry until it is cleared. Crash data is different in kind: written permanently at deployment, unreachable by a scan tool, and removable only through a module reset.
If the low voltage event is over and the supply is healthy now, this code often clears with a scan tool and stays gone, so there is no reason to send a module in for it. Where the reset matters is when the module locked out or is holding crash data alongside the voltage complaint. Fix the charging and ground side first, then call 501-988-0611 if the module still refuses to arm.
This is Mercedes-Benz terminology and a Mercedes-Benz code. Terminal numbering like 15 and 15R is specific to that wiring convention, so the definition does not carry over to other makes even where the code number looks familiar.
Load test the battery and confirm charging voltage before opening a single harness, because most of these trace back to supply rather than to the restraint wiring. Clean and retorque the battery terminals and the main body grounds. Then measure voltage at the module connector with the ignition on and compare it against battery voltage to find any drop. Clear the code and recheck across several drive cycles before condemning the restraint control unit. Related codes: B1002, B1004, B1660, B1024.
Trace the wiring and connectors first. If the module still holds the fault after the repair, mail it to us and we will clear it.
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Do not see it listed? Request a part or call 501-988-0611.