2008 BMW 335xi Power Lumbar Switch Replacement Cost

Power seats are an option in some cars, trucks, and SUVs that allow the front seats to be adjusted to custom positions or shapes for maximum comfort at the push of a button. Besides forward, backward, up, down, tilt, and headrest adjustments, a power seat may also incorporate extra lumbar support for the lower back to prevent driver fatigue and maintain the natural curvature of the spine. This function is controlled with a switch that is mounted either in the door trim panel or on the side of the seat. In many cases, settings for multiple occupants can be memorized by the seat control module (or by the Body Control Module).

The power lumbar switches communicate with the power seat control module that coordinates the function of the power seat motors. When the switch is pressed, the lower region of the seat's backrest moves forward or backward to support the lower back. The mechanism inside the seat might consist of an inflatable chamber or a motor that moves to change the shape of the seat cushion. In some vehicles, the adjustments are memorized by the seat control module. Power seat adjustments, along with dozens of other functions of the vehicle body (power mirrors, locks, windows, airbags, alarm, et. al.) may be overseen by the Body Control Module.

If the power lumbar switch is not working properly, it is likely that the lumbar support will not expand (move forward) or contract (recede backward). Or, the lumbar support might work intermittently. The lumbar support motor or air compressor could be at fault, but the switch is a likely culprit.

A faulty power lumbar switch does not pose an imminent concern with a vehicle. However, improper seat position (relative to the controls and to the airbags) is a safety concern for the driver. If the lumbar support somehow places the driver out of position for safe operation, the problem should be taken care of as soon as possible.

Power seat components are intended to last the lifetime of a vehicle and have no regular replacement interval. The control switches are the most likely components to wear out and fail over time.

To check for a bad power lumbar switch, a technician will first make sure that a fuse has not blown. A blown fuse can be either the cause of an inoperative power lumbar support or a symptom. If all of the seat functions stop working at the same time, it is unlikely to be caused by a bad switch. If all of the seat functions work except the lumbar support, the technician will use a diagnostic meter to check for power to the switch. If voltage is present at the switch, the technician will test the switch for continuity - whether or not it is capable of transmitting electrical current when operated - and possibly bypass the switch to make sure the lumbar support is operational. If the switch has power but will not operate, the switch is likely to be defective. In some cases, the problem might originate in the seat control module or in the Body Control Module. In these cases, the technician will look for a diagnostic trouble code (a fault code) that can be read with a scan tool.

Once a power lumbar switch is found to be the cause of a seat malfunction, a technician will disconnect power to the seat (or disconnect the battery) and carefully remove any interior trim around the switch assembly. The power lumbar switches are most often located on the side of the seat body, but may also be found in another location, such as the door trim panel. With the trim removed, the switch assembly is unplugged from the wiring harness, the switch detached from its housing and replaced with the new switch before reassembly. In most cases, all of the power seat switches are contained in a single assembly that needs to be replaced as a unit, rather than individual switches.

Proper seat alignment is important for safe operation of a vehicle. While a faulty power lumbar switch does not present an immediate danger, it may affect vehicle safety. This applies doubly for long trips as discomfort may cause excessive driver fatigue. It is recommended that a faulty switch be replaced as soon as possible.

It should be noted that some vehicles incorporate Supplemental Restraint System (SRS) features into the seat, such as seat-mounted airbags and/or seatbelt pretensioners. Care is needed to avoid damage to the wires for the SRS and other components located in the seats. The SRS may need to be reset with a special scan tool if, for some reason, the seat is disconnected.

In most cases, a power lumbar switch can be replaced by someone with intermediate DIY experience. But, as is the case with most automotive electrical problems, diagnosis can be the biggest challenge. A fault in the seat control module or the Body Control Module can make it look like the switch is to blame. Repairs can be costly if not diagnosed properly. Access to vehicle-specific information (such as a wiring diagram) and procedures are recommended for an effective repair.