01639: Rear Passenger Side Impact Sensor

Hardware fault

What 01639 means

G257 sits low on the passenger side of the rear body, close enough to the outer skin to sense a side impact early. It has electronics inside it, it runs a self check at every key cycle, and it reports its status back to the restraint controller. A failed self check, an implausible signal, or a status message the controller reads as a defect results in 01639 and a lit airbag lamp.

What separates a hardware entry from a wiring entry is the module's judgment about the part. Here the controller is saying the sensor assembly is not trustworthy. Mounting matters as much as the electronics, since these sensors have to transmit body movement directly through their bolted joint. A sensor hanging loose or shimmed by trim material will report forces that do not match reality.

Common causes

• Loose or missing mounting hardware after quarter panel, door, or rear trim service
• An internal electronic failure inside the G257 sensor
• Moisture and corrosion at the sensor location from a leaking seal or blocked drain
• A non-matching replacement sensor installed in place of the factory unit
• Damaged or spread terminals in the sensor connector
• Body damage near the sensor bracket from an earlier rear or side impact

Crash data or hardware fault?

This entry falls under hardware, which tells you the module suspects the part rather than the path to the part. It is separate from crash data in an important way. Crash data is written permanently when the restraint system fires and cannot be deleted by a scan tool, while a hardware fault reflects the current state of a component and can clear once that component is right.

Does our reset clear it?

Clearing the module while G257 is still faulty accomplishes nothing, since the next self check puts 01639 back. Get the sensor replaced or remounted, then clear. Our mail-in reset is aimed at modules carrying stored deployment data, and it is the right service when the car was in a wreck and the module is locked from that event. Many vehicles that throw rear sensor codes after a collision need both the sensor and the reset.

Which makes throw it

Audi and Volkswagen list 01639 identically, with G257 as the passenger side rear crash sensor. Shared VAG component labeling is why factory documentation from either brand applies.

What to do next

Start with the mounting bolt and the connector, because both are quick to inspect and both cause this code without the sensor being bad. Check that the sensor body contacts bare metal and that no trim clip or sound deadening is trapped underneath. If it is mounted correctly and the fault stays live, plan on a genuine OEM replacement sensor. We stock OEM sensors and can also check the module for stored crash records. Related codes: 01638, 65535, B0091.